PR #336: since we can't use function overriding (due to problems with it on OSX) always include the _json_c_strerror function but only enable it with a flag during tests.

strerror_override_private.h

@@ -0,0 +1,7 @@
+#ifndef __json_strerror_override_private_h__
+#define __json_strerror_override_private_h__
+
+/* Used by tests to get consistent output */
+extern int _json_c_strerror_enable;
+
+#endif