build: simplify libwebp check

Tested (compilation and runtime) with libwebp 0.2.0, 0.2.1 and 0.4.1.

configure

@@ -4860,9 +4860,7 @@ enabled libvpx            && {
     enabled libvpx_vp9_decoder && { check_lib2 "vpx/vpx_decoder.h vpx/vp8dx.h" "vpx_codec_vp9_dx" -lvpx || disable libvpx_vp9_decoder; }
     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           && require_pkg_config libwebp webp/encode.h WebPGetEncoderVersion &&
+enabled libwebp           && require_pkg_config "libwebp >= 0.2.0" webp/encode.h WebPGetEncoderVersion
-                             { check_code cc webp/encode.h "WebPPicture wp; wp.use_argb++" ||
-                               die "ERROR: libwebp too old."; }
 enabled libx264           && require libx264 x264.h x264_encoder_encode -lx264 &&
                              { check_cpp_condition x264.h "X264_BUILD >= 118" ||
                                die "ERROR: libx264 must be installed and version must be >= 0.118."; }