[Backport] handle gnu_source definition for SWIG mapscript (issue #6015)

Author: jmckenna

mapscript/mapscript.i

@@ -33,6 +33,8 @@
 %begin %{
 #ifdef _MSC_VER
 #define SWIG_PYTHON_INTERPRETER_NO_DEBUG
+#else
+#define _GNU_SOURCE 1
 #endif
 %}