vpx decoder: check if library was built with vp8 support

It might only contain the vp9 decoder

configure.ac

@@ -2504,6 +2504,9 @@ AS_IF([test "${enable_vpx}" != "no"],[
         VLC_ADD_PLUGIN([vpx])
         VLC_ADD_CPPFLAGS([vpx], [${VPX_CFLAGS}])
         VLC_ADD_LIBS([vpx], [${VPX_LIBS}])
+        AC_CHECK_LIB([vpx],[vpx_codec_vp8_dx], [
+            VLC_ADD_CPPFLAGS([vpx], [-DENABLE_VP8_DECODER])
+        ], [], [${VPX_LIBS}])
         AC_CHECK_LIB([vpx],[vpx_codec_vp9_dx], [
             VLC_ADD_CPPFLAGS([vpx], [-DENABLE_VP9_DECODER])
         ], [], [${VPX_LIBS}])

modules/codec/vpx.c

@@ -158,10 +158,12 @@ static int Open(vlc_object_t *p_this)
 
     switch (dec->fmt_in.i_codec)
     {
+#ifdef ENABLE_VP8_DECODER
     case VLC_CODEC_VP8:
         iface = &vpx_codec_vp8_dx_algo;
         vp_version = 8;
         break;
+#endif
 #ifdef ENABLE_VP9_DECODER
     case VLC_CODEC_VP9:
         iface = &vpx_codec_vp9_dx_algo;