use our own libpng and libjpeg explicitly.

component/webp/buildout.cfg

+# WebP - A new image format for the Web
+# https://developers.google.com/speed/webp/
+
 [buildout]
 parts =
   webp
+extends =
+  ../libjpeg/buildout.cfg
+  ../libpng/buildout.cfg
 
 [webp]
 recipe = hexagonit.recipe.cmmi
@@ -8,3 +14,7 @@ url = https://webp.googlecode.com/files/libwebp-0.1.3.tar.gz
 md5sum = 254d4670e14e9ed881f0536b006ab336
 configure-options =
   --disable-static
+  --with-jpegincludedir=${libjpeg:location}/include
+  --with-jpeglibdir=${libjpeg:location}/lib
+environment =
+  PATH=${libpng:location}/bin:%(PATH)s