Revert "build-sys: drop -mms-bitfields GCC flag"

This reverts commit 252bbcd2.

After further discussion in !3511 (closed), we’ve decided that there are risks associated with this change, and it’s not the best way of addressing the original problem.

The original motivation for the change turned out to be that -mms-bitfields was not handled by windres, which was receiving it from pkg-config --cflags glib-2.0 in some projects. However, if windres is claiming to accept CFLAGS then it should accept (and ignore) -mms-bitfields, since the -m family of options are defined in man gcc, just like -I, -D, etc.

There is some question that there might still be third party projects which are built with an old enough compiler that -mms-bitfields is not the compiler default. For that reason, we should either still continue to specify -mms-bitfields in the .pc file, or add a test to assert that third party projects are always compiled with -mms-bitfields set. But adding a new test for all third-party compilations is risky (if we get it wrong, things will break; and it’s a test which may behave differently on different platforms), so it seems safer to just keep -mms-bitfields in .pc for now.

Once all compilers which we require specify -mms-bitfields by default, we can finally drop this flag (without adding a test for third-party compilations).

See: !3511 (closed)