Add a dependency check for libwebp
Otherwise the built libgstrswebp.so plugin is built but it doesn't link against libwebp . This result in symbols being left undefined in libgstrswebp.so .
This can be checked by building gst-plugins-rs without libwebp devel files installed in the system. Then running gst-inspect-1.0 gives:
(gst-plugin-scanner-x86_64:31702): GStreamer-WARNING **: 11:41:01.778: Failed to load plugin '/usr/lib64/gstreamer-1.0/libgstrswebp.so': /usr/lib64/gstreamer-1.0/libgstrswebp.so: undefined symbol: WebPAnimDecoderDelete
ldd /usr/lib64/gstreamer-1.0/libgstrswebp.so shows
that the webp plugin doesn't even try to link with the libwebp
This MR fixes that by failing the build when libwebp is not available (and is not disabled using the command line).