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

This reverts commit 252bbcd207.

After further discussion in !3511, 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: https://gitlab.gnome.org/GNOME/glib/-/merge_requests/3511

meson.build

@@ -50,6 +50,7 @@ if cc.get_id() == 'msvc'
   noseh_link_args = ['/SAFESEH:NO']
 else
   noseh_link_args = []
+  # -mms-bitfields vs -fnative-struct ?
 endif
 
 host_system = host_machine.system()
@@ -2351,8 +2352,15 @@ else
   export_dynamic_ldflags = ['-Wl,--export-dynamic']
 endif
 
+win32_cflags = []
 win32_ldflags = []
 if host_system == 'windows' and cc.get_id() != 'msvc' and cc.get_id() != 'clang-cl'
+  # Ensure MSVC-compatible struct packing convention is used when
+  # compiling for Win32 with gcc. It is used for the whole project and exposed
+  # in glib-2.0.pc.
+  win32_cflags = ['-mms-bitfields']
+  add_project_arguments(win32_cflags, language : 'c')
+
   # Win32 API libs, used only by libglib and exposed in glib-2.0.pc
   win32_ldflags = ['-lws2_32', '-lole32', '-lwinmm', '-lshlwapi', '-luuid']
 elif host_system == 'cygwin'