build: require our own ffmpeg repo

This is required now. Can't have FFmpeg upstream randomly break us and
then not fix it (like this recent EOF issue).

Upstream FFmpeg is of course still supported, but you will need to edit
the build scripts. Official support is only with the master branch of
our own repo.

wscript

@@ -462,12 +462,18 @@ libav_dependencies = [
         'req': True,
         'fmsg': "FFmpeg/Libav development files not found.",
     }, {
-        'name': 'is_ffmpeg',
-        'desc': 'libav* is FFmpeg',
+        'name': 'is_ffmpeg_garbage',
+        'desc': 'libav* is upstream FFmpeg (unsupported)',
         # FFmpeg <=> LIBAVUTIL_VERSION_MICRO>=100
         'func': check_statement('libavcodec/version.h',
                                 'int x[LIBAVCODEC_VERSION_MICRO >= 100 ? 1 : -1]',
                                 use='libavcodec')
+    }, {
+        'name': 'is_ffmpeg',
+        'desc': 'libav* is FFmpeg mpv modified version',
+        'func': check_statement('libavcodec/version.h',
+                                'int x[LIBAVCODEC_MPV ? 1 : -1]',
+                                use='libavcodec')
     }, {
         # This check should always result in the opposite of is_ffmpeg.
         # Run it to make sure is_ffmpeg didn't fail for some other reason than
@@ -485,7 +491,9 @@ libav_dependencies = [
         'func': check_ffmpeg_or_libav_versions(),
         'req': True,
         'fmsg': "Unable to find development files for some of the required \
-FFmpeg/Libav libraries. You need at least {0}. Aborting.".format(libav_versions_string)
+FFmpeg/Libav libraries. You need at least {0}. For FFmpeg, the mpv fork, that \
+might contain additional fixes and features is required. It is available on \
+https://github.com/mpv-player/ffmpeg-mpv Aborting.".format(libav_versions_string)
     }, {
         'name': '--libavdevice',
         'desc': 'libavdevice',