Vid.stab: Build for CLANG* environments

[mmuetzel]

Also use it in ffmpeg in these environments.

[mmuetzel]

Re-based to current HEAD and squashed some of the commits.

[mmuetzel]

Compiling ffmpeg in MINGW32 now fails with:

  In file included from C:/_/mingw-w64-ffmpeg/src/ffmpeg-4.4.2/libavcodec/cabac_functions.h:47,
                   from C:/_/mingw-w64-ffmpeg/src/ffmpeg-4.4.2/libavcodec/h264_cabac.c:36:
  In function 'get_cabac_inline_x86',
      inlined from 'get_cabac' at C:/_/mingw-w64-ffmpeg/src/ffmpeg-4.4.2/libavcodec/cabac_functions.h:140:12,
      inlined from 'decode_cabac_mb_intra4x4_pred_mode' at C:/_/mingw-w64-ffmpeg/src/ffmpeg-4.4.2/libavcodec/h264_cabac.c:1368:9,
      inlined from 'ff_h264_decode_mb_cabac' at C:/_/mingw-w64-ffmpeg/src/ffmpeg-4.4.2/libavcodec/h264_cabac.c:2072:32:
  C:/_/mingw-w64-ffmpeg/src/ffmpeg-4.4.2/libavcodec/x86/cabac.h:199:5: error: 'asm' operand has impossible constraints
    199 |     __asm__ volatile(
        |     ^~~~~~~
  C:/_/mingw-w64-ffmpeg/src/ffmpeg-4.4.2/libavcodec/x86/cabac.h:199:5: error: 'asm' operand has impossible constraints
  C:/_/mingw-w64-ffmpeg/src/ffmpeg-4.4.2/libavcodec/x86/cabac.h:199:5: error: 'asm' operand has impossible constraints
  C:/_/mingw-w64-ffmpeg/src/ffmpeg-4.4.2/libavcodec/x86/cabac.h:199:5: error: 'asm' operand has impossible constraints
  make: *** [/C/_/mingw-w64-ffmpeg/src/ffmpeg-4.4.2/ffbuild/common.mak:67: libavcodec/h264_cabac.o] Error 1
  make: *** Waiting for unfinished jobs....

I didn't see that error in the CI before. Maybe something new with GCC 12.1?

[mmuetzel]

I added a patch that de-activates the assembler code that doesn't compile on MINGW32 with GCC 12.1. In that case, the implementation falls back to C code which might be less efficient. But at least, it seems to be compiling with that change.