AviSynth demuxer rewrite.

Dynamically loads the library itself, rather than through VfW.
Supports AvxSynth on Linux and OS X.

Supports the new colorspaces added in AviSynth 2.6 when used
with AviSynth 2.6.

configure

@@ -3917,7 +3917,9 @@ for func in $MATH_FUNCS; do
 done
 
 # these are off by default, so fail if requested and not available
-enabled avisynth   && require2 vfw32 "windows.h vfw.h" AVIFileInit -lavifil32
+enabled avisynth   && { { check_lib2 "windows.h" LoadLibrary; } ||
+                        { check_lib2 "dlfcn.h" dlopen -ldl; } ||
+                        die "ERROR: LoadLibrary/dlopen not found for avisynth"; }
 enabled fontconfig && require_pkg_config fontconfig "fontconfig/fontconfig.h" FcInit
 enabled frei0r     && { check_header frei0r.h || die "ERROR: frei0r.h header not found"; }
 enabled gnutls     && require_pkg_config gnutls gnutls/gnutls.h gnutls_global_init