configure: simplify libwebp checks

The libwebpmux pkg-config file already has an explicit dependecy on libwebp >= 0.2.0.
Also remove the warning and silently disable the anim encoder when libwebpmux is not new enough.
This is more in line with other library components, like libvpx-vp9

Reviewed-by: Michael Niedermayer <michaelni@gmx.at>
Signed-off-by: James Almer <jamrial@gmail.com>

configure

@@ -5104,10 +5104,8 @@ enabled libvpx            && {
     enabled libvpx_vp9_encoder && { check_lib2 "vpx/vpx_encoder.h vpx/vp8cx.h" "vpx_codec_vp9_cx VP9E_SET_AQ_MODE" -lvpx || disable libvpx_vp9_encoder; } }
 enabled libwavpack        && require libwavpack wavpack/wavpack.h WavpackOpenFileOutput  -lwavpack
 enabled libwebp           && {
-    enabled libwebp_encoder && require_pkg_config "libwebp >= 0.2.0" webp/encode.h WebPGetEncoderVersion;
-    enabled libwebp_anim_encoder && require_pkg_config "libwebp >= 0.2.0" webp/encode.h WebPGetEncoderVersion &&
-                                    { use_pkg_config "libwebpmux >= 0.4.0" webp/mux.h WebPAnimEncoderOptionsInit ||
-                                      { disable libwebp_anim_encoder && warn "using libwebp without libwebpmux"; } } }
+    enabled libwebp_encoder      && require_pkg_config "libwebp >= 0.2.0" webp/encode.h WebPGetEncoderVersion
+    enabled libwebp_anim_encoder && { use_pkg_config "libwebpmux >= 0.4.0" webp/mux.h WebPAnimEncoderOptionsInit || disable libwebp_anim_encoder; } }
 enabled libx264           && { use_pkg_config x264 "stdint.h x264.h" x264_encoder_encode ||
                                { require libx264 x264.h x264_encoder_encode -lx264 &&
                                  warn "using libx264 without pkg-config"; } } &&