Ignore function pointer cast warnings

Use GCC pragmas as these warnings are enabled by -Wpedantic and there's
no way to disable them selectively.

libxml.h

@@ -60,6 +60,18 @@ int vfprintf(FILE *, const char *, va_list);
 #include "trio.h"
 #endif
 
+#if defined(__clang__) || \
+    (defined(__GNUC__) && (__GNUC__ * 100 + __GNUC_MINOR__ >= 406))
+#define XML_IGNORE_PEDANTIC_WARNINGS \
+    _Pragma("GCC diagnostic push") \
+    _Pragma("GCC diagnostic ignored \"-Wpedantic\"")
+#define XML_POP_WARNINGS \
+    _Pragma("GCC diagnostic pop")
+#else
+#define XML_IGNORE_PEDANTIC_WARNINGS
+#define XML_POP_WARNINGS
+#endif
+
 /*
  * Internal variable indicating if a callback has been registered for
  * node creation/destruction. It avoids spending a lot of time in locking