silence webp -Wunused-function warning

include/mapnik/webp_io.hpp

@@ -30,7 +30,10 @@
 // webp
 #pragma clang diagnostic push
 #pragma clang diagnostic ignored "-Wunused-function"
+extern "C"
+{
 #include <webp/encode.h>
+}
 #pragma clang diagnostic pop
 
 // stl

src/webp_reader.cpp

@@ -25,11 +25,14 @@
 #include <mapnik/debug.hpp>
 #include <mapnik/image_reader.hpp>
 
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wunused-function"
 extern "C"
 {
 #include <webp/types.h>
 #include <webp/decode.h>
 }
+#pragma clang diagnostic pop
 
 // boost
 #include <boost/iostreams/device/file.hpp>