FYI: Compiler warnings that appear on stderr (skipping any messages sent to stdout) - xlc V11 as the reporting compiler.

Two that may need attention are the redefines of _POSIX_C_SOURCE and _XOPEN_SOURCE in pyconfig.h

Also surprised by the message:
Could not find platform dependent libraries <exec_prefix>
Consider setting $PYTHONHOME to <prefix>[:<exec_prefix>]

I do not recall seeing that message in other builds.

Note also: no messages (to stderr) from configure

Details:

+ CPPFLAGS="-I/opt/include -I/opt/buildaix/include" CFLAGS="-I/opt/include -qmaxmem=-1 -qarch=pwr4 -O2 -I/opt/buildaix/includes"\
        ./configure\
                --prefix=/opt \
                --sysconfdir=/var/python/etc\
                --sharedstatedir=/var/python/com\
                --localstatedir=/var/python\
                --mandir=/usr/share/man\
                --infodir=/opt/share/info/python --without-computed-gotos\
                        > .buildaix/configure.out
+ make > .buildaix/make.out
"Objects/moduleobject.c", line 241.20: 1506-068 (W) Operation between types "struct _object*(*)(struct _object*,struct PyModuleDef*)" and "void*" is not allowed.
"./pyconfig.h", line 1472.9: 1506-236 (W) Macro name _POSIX_C_SOURCE has been redefined.
"./pyconfig.h", line 1472.9: 1506-358 (I) "_POSIX_C_SOURCE" is defined on line 147 of /usr/include/standards.h.
"./pyconfig.h", line 1494.9: 1506-236 (W) Macro name _XOPEN_SOURCE has been redefined.
"./pyconfig.h", line 1494.9: 1506-358 (I) "_XOPEN_SOURCE" is defined on line 143 of /usr/include/standards.h.
"./Modules/posixmodule.c", line 5532.11: 1506-131 (W) Explicit dimension specification or initializer required for an auto or static array.
"./Modules/xxsubtype.c", line 293.19: 1506-196 (W) Initialization between types "void*" and "int(*)(struct _object*)" is not allowed.
nm: libpython3.6m.a: 0654-200 Cannot open the specified file.
nm: A file or directory in the path name does not exist.
Could not find platform dependent libraries <exec_prefix>
Consider setting $PYTHONHOME to <prefix>[:<exec_prefix>]
Could not find platform dependent libraries <exec_prefix>
Consider setting $PYTHONHOME to <prefix>[:<exec_prefix>]
"/data/prj/aixtools/python/python-3.6.0.164/Modules/_ctypes/_ctypes_test.c", line 392.5: 1506-159 (E) Bit field type specified for M is not valid. Type unsigned assumed.
"/data/prj/aixtools/python/python-3.6.0.164/Modules/_ctypes/_ctypes_test.c", line 392.5: 1506-159 (E) Bit field type specified for N is not valid. Type unsigned assumed.
"/data/prj/aixtools/python/python-3.6.0.164/Modules/_ctypes/_ctypes_test.c", line 392.5: 1506-159 (E) Bit field type specified for O is not valid. Type unsigned assumed.
"/data/prj/aixtools/python/python-3.6.0.164/Modules/_ctypes/_ctypes_test.c", line 392.5: 1506-159 (E) Bit field type specified for P is not valid. Type unsigned assumed.
"/data/prj/aixtools/python/python-3.6.0.164/Modules/_ctypes/_ctypes_test.c", line 392.5: 1506-159 (E) Bit field type specified for Q is not valid. Type unsigned assumed.
"/data/prj/aixtools/python/python-3.6.0.164/Modules/_ctypes/_ctypes_test.c", line 392.5: 1506-159 (E) Bit field type specified for R is not valid. Type unsigned assumed.
"/data/prj/aixtools/python/python-3.6.0.164/Modules/_ctypes/_ctypes_test.c", line 392.5: 1506-159 (E) Bit field type specified for S is not valid. Type unsigned assumed.
"/data/prj/aixtools/python/python-3.6.0.164/Modules/arraymodule.c", line 2996.19: 1506-196 (W) Initialization between types "void*" and "int(*)(struct _object*)" is not allowed.
"/data/prj/aixtools/python/python-3.6.0.164/Modules/_testmultiphase.c", line 84.22: 1506-196 (W) Initialization between types "void*" and "int(*)(struct {...}*)" is not allowed.
"/data/prj/aixtools/python/python-3.6.0.164/Modules/_testmultiphase.c", line 85.22: 1506-196 (W) Initialization between types "void*" and "int(*)(struct {...}*,int(*)(struct _object*,void*),void*)" is not allowed.
"/data/prj/aixtools/python/python-3.6.0.164/Modules/_testmultiphase.c", line 86.22: 1506-196 (W) Initialization between types "void*" and "struct _object*(*)(struct {...}*,struct _object*)" is not allowed.
"/data/prj/aixtools/python/python-3.6.0.164/Modules/_testmultiphase.c", line 87.21: 1506-196 (W) Initialization between types "void*" and "int(*)(struct {...}*,const char*,struct _object*)" is not allowed.
"/data/prj/aixtools/python/python-3.6.0.164/Modules/_testmultiphase.c", line 235.19: 1506-196 (W) Initialization between types "void*" and "int(*)(struct _object*)" is not allowed.
"/data/prj/aixtools/python/python-3.6.0.164/Modules/_testmultiphase.c", line 281.21: 1506-196 (W) Initialization between types "void*" and "struct _object*(*)(struct _object*,struct PyModuleDef*)" is not allowed.
"/data/prj/aixtools/python/python-3.6.0.164/Modules/_testmultiphase.c", line 454.21: 1506-196 (W) Initialization between types "void*" and "struct _object*(*)(struct _object*,struct PyModuleDef*)" is not allowed.
"/data/prj/aixtools/python/python-3.6.0.164/Modules/_testmultiphase.c", line 475.21: 1506-196 (W) Initialization between types "void*" and "struct _object*(*)(struct _object*,struct PyModuleDef*)" is not allowed.
"/data/prj/aixtools/python/python-3.6.0.164/Modules/_testmultiphase.c", line 496.21: 1506-196 (W) Initialization between types "void*" and "struct _object*(*)(struct _object*,struct PyModuleDef*)" is not allowed.
"/data/prj/aixtools/python/python-3.6.0.164/Modules/_testmultiphase.c", line 510.21: 1506-196 (W) Initialization between types "void*" and "struct _object*(*)(struct _object*,struct PyModuleDef*)" is not allowed.
"/data/prj/aixtools/python/python-3.6.0.164/Modules/_testmultiphase.c", line 511.19: 1506-196 (W) Initialization between types "void*" and "int(*)(struct _object*)" is not allowed.
"/data/prj/aixtools/python/python-3.6.0.164/Modules/_testmultiphase.c", line 531.19: 1506-196 (W) Initialization between types "void*" and "int(*)(struct _object*)" is not allowed.
"/data/prj/aixtools/python/python-3.6.0.164/Modules/_testmultiphase.c", line 552.19: 1506-196 (W) Initialization between types "void*" and "int(*)(struct _object*)" is not allowed.
"/data/prj/aixtools/python/python-3.6.0.164/Modules/_testmultiphase.c", line 573.19: 1506-196 (W) Initialization between types "void*" and "int(*)(struct _object*)" is not allowed.
"/data/prj/aixtools/python/python-3.6.0.164/Modules/_ssl.c", line 302.17: 1506-196 (W) Initialization between types "void*" and "struct _object*(*)(struct {...}*)" is not allowed.
"/data/prj/aixtools/python/python-3.6.0.164/Modules/pyexpat.c", line 1358.27: 1506-068 (W) Operation between types "void*" and "void(*)(void*,const char*,int)" is not allowed.
"./pyconfig.h", line 1472.9: 1506-236 (W) Macro name _POSIX_C_SOURCE has been redefined.
"./pyconfig.h", line 1472.9: 1506-358 (I) "_POSIX_C_SOURCE" is defined on line 147 of /usr/include/standards.h.
"./pyconfig.h", line 1494.9: 1506-236 (W) Macro name _XOPEN_SOURCE has been redefined.
"./pyconfig.h", line 1494.9: 1506-358 (I) "_XOPEN_SOURCE" is defined on line 143 of /usr/include/standards.h.
"./pyconfig.h", line 1472.9: 1506-236 (W) Macro name _POSIX_C_SOURCE has been redefined.
"./pyconfig.h", line 1472.9: 1506-358 (I) "_POSIX_C_SOURCE" is defined on line 147 of /usr/include/standards.h.
"./pyconfig.h", line 1494.9: 1506-236 (W) Macro name _XOPEN_SOURCE has been redefined.
"./pyconfig.h", line 1494.9: 1506-358 (I) "_XOPEN_SOURCE" is defined on line 143 of /usr/include/standards.h.
"/data/prj/aixtools/python/python-3.6.0.164/Modules/xxlimited.c", line 112.22: 1506-196 (W) Initialization between types "void*" and "int(*)(struct {...}*,int(*)(struct _object*,void*),void*)" is not allowed.
"/data/prj/aixtools/python/python-3.6.0.164/Modules/xxlimited.c", line 113.22: 1506-196 (W) Initialization between types "void*" and "int(*)(struct {...}*)" is not allowed.
"/data/prj/aixtools/python/python-3.6.0.164/Modules/xxlimited.c", line 114.22: 1506-196 (W) Initialization between types "void*" and "struct _object*(*)(struct {...}*,struct _object*)" is not allowed.
"/data/prj/aixtools/python/python-3.6.0.164/Modules/xxlimited.c", line 115.21: 1506-196 (W) Initialization between types "void*" and "int(*)(struct {...}*,const char*,struct _object*)" is not allowed.
"/data/prj/aixtools/python/python-3.6.0.164/Modules/xxlimited.c", line 204.25: 1506-196 (W) Initialization between types "void*" and "struct _object*(*)(struct _object*,struct _object*,int)" is not allowed.
"/data/prj/aixtools/python/python-3.6.0.164/Modules/xxlimited.c", line 242.30: 1506-068 (W) Operation between types "void*" and "struct _object*(*)(struct _typeobject*,struct _object*,struct _object*)" is not allowed.
"/data/prj/aixtools/python/python-3.6.0.164/Modules/xxlimited.c", line 283.19: 1506-196 (W) Initialization between types "void*" and "int(*)(struct _object*)" is not allowed.
"/data/prj/aixtools/python/python-3.6.0.164/Modules/_ctypes/_ctypes.c", line 3324.27: 1506-068 (W) Operation between types "void*" and "int(*)(void)" is not allowed.
"/data/prj/aixtools/python/python-3.6.0.164/Modules/_ctypes/_ctypes.c", line 3869.31: 1506-280 (W) Function argument assignment between types "int(*)(void)" and "void*" is not allowed.
"/data/prj/aixtools/python/python-3.6.0.164/Modules/_ctypes/_ctypes.c", line 5482.63: 1506-280 (W) Function argument assignment between types "void*" and "void*(*)(void*,const void*,unsigned long)" is not allowed.
"/data/prj/aixtools/python/python-3.6.0.164/Modules/_ctypes/_ctypes.c", line 5483.62: 1506-280 (W) Function argument assignment between types "void*" and "void*(*)(void*,int,unsigned long)" is not allowed.
"/data/prj/aixtools/python/python-3.6.0.164/Modules/_ctypes/_ctypes.c", line 5484.65: 1506-280 (W) Function argument assignment between types "void*" and "struct _object*(*)(const char*,int)" is not allowed.
"/data/prj/aixtools/python/python-3.6.0.164/Modules/_ctypes/_ctypes.c", line 5485.60: 1506-280 (W) Function argument assignment between types "void*" and "struct _object*(*)(void*,struct _object*,struct _object*)" is not allowed.
"/data/prj/aixtools/python/python-3.6.0.164/Modules/_ctypes/_ctypes.c", line 5487.66: 1506-280 (W) Function argument assignment between types "void*" and "struct _object*(*)(const unsigned int*,int)" is not allowed.
"/data/prj/aixtools/python/python-3.6.0.164/Modules/_ctypes/callproc.c", line 813.32: 1506-280 (W) Function argument assignment between types "void(*)(void)" and "void*" is not allowed.
+ make install DESTDIR=/var/aixtools/aixtools/python/3.6.0.164 > .buildaix/install.out
+ mkinstallp.ksh /var/aixtools/aixtools/python/3.6.0.164 > .buildaix/mkinstallp.out
renamed:./opt/lib/python3.6/site-packages/setuptools/script (dev).tmpl:./opt/lib/python3.6/site-packages/setuptools/script_dev_.tmpl
renamed:./opt/lib/python3.6/site-packages/setuptools/command/launcher manifest.xml:./opt/lib/python3.6/site-packages/setuptools/command/launcher_manifest.xml

The warnings about platform-dependent libraries should be suppressed now thanks to Issue 27713.

The warnings from Modules/_ctypes/_ctypes_test.c about bitfields are covered by Issue 27643. Can you help with developing the patch?

The remaning warnings all seem to be duplicates of Issue 28290.