configure: check avisynth header version

The headers from version 3.7.1 are needed in order to support
parsing of frame properties. avs/version.h has been generated
as part of the AviSynth+ build process for a long time, but was
never installed with the includes until version 3.7.1a. Checking
for the presence of avs/version.h might have been sufficient,
but a version check mechanism might be useful in the future.

This does not change the version compatibility with the library
itself; previous 3.x versions of AviSynth+ as well as AviSynth 2.6
can still be used with the demuxer.

Signed-off-by: Stephen Hutchinson <qyot27@gmail.com>

configure

@@ -6501,7 +6501,9 @@ for func in $COMPLEX_FUNCS; do
 done
 
 # these are off by default, so fail if requested and not available
-enabled avisynth          && require_headers "avisynth/avisynth_c.h"
+enabled avisynth          && { require_headers "avisynth/avisynth_c.h avisynth/avs/version.h" &&
+                               { test_cpp_condition avisynth/avs/version.h "AVS_MAJOR_VER >= 3 && AVS_MINOR_VER >= 7 && AVS_BUGFIX_VER >= 1 || AVS_MAJOR_VER >= 3 && AVS_MINOR_VER > 7 || AVS_MAJOR_VER > 3" ||
+                                 die "ERROR: AviSynth+ header version must be >= 3.7.1"; } }
 enabled cuda_nvcc         && { check_nvcc cuda_nvcc || die "ERROR: failed checking for nvcc."; }
 enabled chromaprint       && require chromaprint chromaprint.h chromaprint_get_version -lchromaprint
 enabled decklink          && { require_headers DeckLinkAPI.h &&