As part of the update, it is now possible to test 32-bit GCC builds
of AviSynth+ with FFmpeg by using the AVS_WIN32_GCC32 define. Due to
different calling conventions between MSVC and GCC regarding 32-bit
Windows, this is unfortunately necessary.
The demuxer doesn't use these functions, so it shouldn't affect
anything. Investigate whether this can be fixed by how headers
are checked in configure.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>