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.

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

If the user attempts to use AviSynth 2.5, an error message will
now tell them they need to upgrade.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

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>

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

Fixes: CID1257658
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

Correct the FSF address for two avisynth files to '51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA'
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

Uses the 2.5 compatibility header included with the variant of
FFMS2 that uses AviSynth's C-interface. A copy of this header is
now provided in compat/avisynth.

avs_get_row_size_p and avs_get_height_p changed between versions
2.5 and 2.6. Since the avisynth_c.h header that avformat uses
assumes AviSynth 2.6, it would cause 2.5 to crash if given any
kind of real video (the Version() function was known to work,
though).

AvxSynth was unaffected by this issue because, despite being based
on AviSynth 2.5.8 and using 2.5.8's interface version number of 3,
it actually uses 2.6's versions of these functions.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

The versions of the headers are the same as those provided with x264
for consistency's sake.