We skipped adding it to cflags for mingw/cygwin configurations
where the compiler either complains loudly or even errors out;
do the same for target_os=win32, for the case when building with
clang in msvc mode.

This wasn't needed for the actual msvc before, since msvc uses
msvc_common_flags to filter out this flag from cflags.
When building with clang in msvc mode, no such filter is used.

This fixes building shared libraries with clang in msvc mode.
Signed-off-by: Martin Storsjö <martin@martin.st>