avisynth: Bump minimum required version to interface version 6

The AVSC_API changes in the new headers mean that the 2.6 alphas
are just as incompatible as 2.5 is.
Signed-off-by: Diego Biurrun <diego@biurrun.de>

doc/general.texi

@@ -151,8 +151,8 @@ by Libav. Once you have the appropriate header, pass
 @code{--enable-avisynth} to configure to enable AviSynth support.
 
 For Windows, supported AviSynth variants are
-@url{http://avisynth.nl, AviSynth 2.6} for 32-bit builds and
-@url{http://avs-plus.net, AviSynth+ 0.1} for 32-bit and 64-bit builds.
+@url{http://avisynth.nl, AviSynth 2.6 RC1 or higher} for 32-bit builds and
+@url{http://avs-plus.net, AviSynth+ r1718 or higher} for 32-bit and 64-bit builds.
 @url{https://github.com/AviSynth/AviSynthPlus, AviSynth+'s git repository}
 provides a GNU-style Makefile which can install just the headers using
 @code{make install PREFIX=/install/prefix}.

libavformat/avisynth.c

@@ -407,13 +407,14 @@ static int avisynth_open_file(AVFormatContext *s)
     avs->vi   = avs_library.avs_get_video_info(avs->clip);
 
 #ifdef USING_AVISYNTH
-    /* libav only supports AviSynth 2.6 on Windows. Since AvxSynth
-     * identifies itself as interface version 3 like 2.5.8, this
-     * needs to be special-cased. */
+    /* On Windows, libav supports AviSynth interface version 6 or higher.
+     * This includes AviSynth 2.6 RC1 or higher, and AviSynth+ r1718 or higher,
+     * and excludes 2.5 and the 2.6 alphas. Since AvxSynth identifies itself
+     * as interface version 3 like 2.5.8, this needs to be special-cased. */
 
-    if (avs_library.avs_get_version(avs->clip) == 3) {
+    if (avs_library.avs_get_version(avs->clip) < 6) {
         av_log(s, AV_LOG_ERROR,
-               "AviSynth 2.5.8 not supported. Please upgrade to 2.6.\n");
+               "AviSynth version is too old. Please upgrade to either AviSynth 2.6 >= RC1 or AviSynth+ >= r1718.\n");
         ret = AVERROR_UNKNOWN;
         goto fail;
     }