While compiling from sources I see in the process :

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creating build/temp.solaris-2.10-sun4v.64bit-3.5/libffi
checking build system type... sparc-sun-solaris2.10
checking host system type... sparc-sun-solaris2.10
checking target system type... sparc-sun-solaris2.10
checking for gsed... /usr/local/bin/gsed
checking for a BSD-compatible install... /usr/local/build/python-3.5.1_SunOS5.10_sparcv9.002/Modules/_ctypes/libffi/install-sh -c
checking whether build environment is sane... yes
checking for a thread-safe mkdir -p... /usr/local/build/python-3.5.1_SunOS5.10_sparcv9.002/Modules/_ctypes/libffi/install-sh -c -d
checking for gawk... /usr/local/bin/gawk
checking whether /usr/local/bin/gmake sets $(MAKE)... yes
checking whether /usr/local/bin/gmake supports nested variables... yes
checking for gcc... /opt/solarisstudio12.4/bin/cc
checking whether the C compiler works... no
configure: error: in `/usr/local/build/python-3.5.1_SunOS5.10_sparcv9.002/build/temp.solaris-2.10-sun4v.64bit-3.5/libffi':
configure: error: C compiler cannot create executables
See `config.log' for more details


The reason for this error is that the build process down in the libffi directory seems to spawn its own "configure" stage wherein the CFLAGS are lost, forgotten or simply not used and therefore a trivial compile fails because of :

configure:3873: /opt/solarisstudio12.4/bin/cc  -I/usr/local/include -I/usr/local/ssl/include -D_TS_ERRNO -D_POSIX_PTHREAD_SEMANTICS -D_LARGEFILE64_SOURCE  conftest.c  >&5
ld: fatal: file /opt/solarisstudio12.4/lib/compilers/crti.o: wrong ELF class: ELFCLASS32
ld: fatal: file processing errors. No output written to a.out
configure:3877: $? = 2
configure:3915: result: no
configure: failed program was:
| /* confdefs.h */
| #define PACKAGE_NAME "libffi"
| #define PACKAGE_TARNAME "libffi"
| #define PACKAGE_VERSION "3.1"
| #define PACKAGE_STRING "libffi 3.1"
| #define PACKAGE_BUGREPORT "http://github.com/atgreen/libffi/issues"
| #define PACKAGE_URL ""
| #define PACKAGE "libffi"
| #define VERSION "3.1"
| /* end confdefs.h.  */
|
| int
| main ()
| {
|
|   ;
|   return 0;
| }
configure:3920: error: in `/usr/local/build/python-3.5.1_SunOS5.10_sparcv9.002/build/temp.solaris-2.10-sun4v.64bit-3.5/libffi':
configure:3922: error: C compiler cannot create executables
See `config.log' for more details


The configure process MUST use the CFLAGS that were passed to the original configure stage and reside in the Makefile.  The absence of the flag -m64 causes a 32-bit compile here and thus the compile fails.