configure: allow enabling libvpx vp9 modules when vp8 is disabled

libvpx can be compiled with the VP8 decoder and encoder disabled, and
there's no reason to force their presence if the user only wants VP9.
Signed-off-by: James Almer <jamrial@gmail.com>

configure

@@ -6226,13 +6226,11 @@ enabled libvorbis         && require_pkg_config libvorbis vorbis vorbis/codec.h
 enabled libvpx            && {
     enabled libvpx_vp8_decoder && {
         check_pkg_config libvpx_vp8_decoder "vpx >= 1.4.0" "vpx/vpx_decoder.h vpx/vp8dx.h" vpx_codec_vp8_dx ||
-            check_lib libvpx_vp8_decoder "vpx/vpx_decoder.h vpx/vp8dx.h" "vpx_codec_vp8_dx VPX_IMG_FMT_HIGHBITDEPTH" "-lvpx $libm_extralibs $pthreads_extralibs" ||
-                die "ERROR: libvpx decoder version must be >=1.4.0";
+            check_lib libvpx_vp8_decoder "vpx/vpx_decoder.h vpx/vp8dx.h" "vpx_codec_vp8_dx VPX_IMG_FMT_HIGHBITDEPTH" "-lvpx $libm_extralibs $pthreads_extralibs"
     }
     enabled libvpx_vp8_encoder && {
         check_pkg_config libvpx_vp8_encoder "vpx >= 1.4.0" "vpx/vpx_encoder.h vpx/vp8cx.h" vpx_codec_vp8_cx ||
-            check_lib libvpx_vp8_encoder "vpx/vpx_encoder.h vpx/vp8cx.h" "vpx_codec_vp8_cx VPX_IMG_FMT_HIGHBITDEPTH" "-lvpx $libm_extralibs $pthreads_extralibs" ||
-                die "ERROR: libvpx encoder version must be >=1.4.0";
+            check_lib libvpx_vp8_encoder "vpx/vpx_encoder.h vpx/vp8cx.h" "vpx_codec_vp8_cx VPX_IMG_FMT_HIGHBITDEPTH" "-lvpx $libm_extralibs $pthreads_extralibs"
     }
     enabled libvpx_vp9_decoder && {
         check_pkg_config libvpx_vp9_decoder "vpx >= 1.4.0" "vpx/vpx_decoder.h vpx/vp8dx.h" vpx_codec_vp9_dx ||