GCC and IBM xlC compilers pay attention to different things - so some of the messages are "accepted" if not harmless.

e.g. this type:
"./Modules/xxsubtype.c", line 293.19: 1506-196 (W) Initialization between types "void*" and "int(*)(struct _object*)" is not allowed.

Others are (well)-known different compiler behaviors - such as the default sign of a 'bit-field'
"/data/prj/python/python-3.6.0.177/Modules/_ctypes/_ctypes_test.c", line 389.5: 1506-159 (E) Bit field type specified for M is not valid. Type unsigned assumed.

However, some of the messages may lead to a problem down the road. Might be the platform, might be the "converted" include files:

"./pyconfig.h", line 1448.9: 1506-236 (W) Macro name _POSIX_C_SOURCE has been redefined.
"./pyconfig.h", line 1448.9: 1506-358 (I) "_POSIX_C_SOURCE" is defined on line 147 of /usr/include/standards.h.
"./pyconfig.h", line 1460.9: 1506-236 (W) Macro name _XOPEN_SOURCE has been redefined.
"./pyconfig.h", line 1460.9: 1506-358 (I) "_XOPEN_SOURCE" is defined on line 143 of /usr/include/standards.h.

And others might be more of an issue - e.g., no idea what "_blake2" is - does not look 'happy' or safe:
"/data/prj/python/python-3.6.0.177/Modules/_blake2/impl/blake2.h", line 89.14: 1506-508 (W) Option push for pragma pack is not supported.
"/data/prj/python/python-3.6.0.177/Modules/_blake2/impl/blake2.h", line 89.1: 1506-224 (W) Incorrect pragma ignored.
"/data/prj/python/python-3.6.0.177/Modules/_blake2/impl/blake2.h", line 119.14: 1506-726 (W) Attempting to pop an empty pack stack. Current pack may change.

That pretty well summarizes what is in the file - stdout was directed to a file