Compiler and linker flags for _decimal and internal libmpdec are currently handled by a mix of configure checks and if/else chains in setup.py. The split makes it harder to build _decimal correctly from Modules/Setup. The Modules/Setup file also does not handle --with-system-mpdec.

I have a working PR that moves all logic into configure.ac. The new system:

* sets LIBMPDEC_CFLAGS and LIBMPDEC_LDFLAGS based on --with-system-libmpdec value.

* detects libmpdec_machine by looking at ac_sys_system, MACOSX_DEFAULT_ARCH, ac_cv_sizeof_size_t, ac_cv_gcc_asm_for_x64, ac_cv_type___uint128_t, and ac_cv_gcc_asm_for_x87.

* sets libmpdec compiler args based on libmpdec_machine, have_ipa_pure_const_bug, and have_glibc_memmove_bug.

* if --with-system-libmpdec is not given, then our Makefile compiles libmpdec objects and puts them into a libmpdec.a archive.

* finally it either links _decimal with our libmpdec.a or with system's libmpdec shared library.

I went for libmpdec.a because it makes the logic for the internal path look similar to the logic with linking with an external shared library.

Modules/Setup