How to compile graphicsmagick with webp support

So I'm trying to compile graphicsmagick with webp support... I've run

yum install libwebp libwebp-dev

I've added the libwebp.a library from google . And I add the --with-webp flag to the ./configure line. I've managed to take it from:

checking for WEBP support ... 
checking webp/decode.h usability... no
checking webp/decode.h presence... no
checking for webp/decode.h... no
checking webp/encode.h usability... no
checking webp/encode.h presence... no
checking for webp/encode.h... no
checking for WebPDecodeRGB in -lwebp... no
checking for WebPMuxSetImage in -lwebpmux... no
checking if WEBP package is complete... no -- some components failed test

To

checking for WEBP support ... 
checking webp/decode.h usability... no
checking webp/decode.h presence... no
checking for webp/decode.h... no
checking webp/encode.h usability... no
checking webp/encode.h presence... no
checking for webp/encode.h... no
checking for WebPDecodeRGB in -lwebp... yes
checking for WebPMuxSetImage in -lwebpmux... no
checking if WEBP package is complete... no -- some components failed test

So only the checking for WebPDecodeRGB in -lwebp has changed. What do I do?