It doesn't work with GCC reverting back "disable [-Wunused-result] warning GCC : https://gcc.gnu.org/bugzilla/show_bug.cgi?id=25509"

This reverts commit 3c76e8d7ef.

include/mapnik/util/file_io.hpp

@@ -83,11 +83,7 @@ public:
         if (!size_) return nullptr;
         std::fseek(file_.get(), 0, SEEK_SET);
         data_type buffer(new char[size_]);
-#pragma GCC diagnostic push
-#pragma GCC diagnostic ignored "-Wunknown-pragmas"
-#pragma GCC diagnostics ignored "-Wunused-result"
         std::fread(buffer.get(), size_, 1, file_.get());
-#pragma GCC diagnostic pop
         return buffer;
     }
 private: