Revert "Revert WebP support due to broken builds"

This reverts commit 6709bdcacd.

There was an issue with the script that removes '-lwebp' from WebP
linker flags, since we're statically linking. This is now fixed.

platform/linux/scripts/configure.sh

@@ -15,6 +15,7 @@ VARIANT_VERSION=1.0
 RAPIDJSON_VERSION=1.0.2
 GTEST_VERSION=1.7.0
 PIXELMATCH_VERSION=0.9.0
+WEBP_VERSION=0.5.0
 
 function print_opengl_flags {
     CONFIG+="    'opengl_cflags%': $(quote_flags $(pkg-config gl x11 --cflags)),"$LN