build: make strerror() override-able

If we want to override `strerror()` in libjson-c
to make tests consistent across platforms, we
need to do it build-wide as configure/build
option.

Apple linkers make it really hard to override functions
at link-time, and this seems to be locked down on travis-ci.org
[ for security reasons I assume ].
While I got it to work locally, it did not work
when running on travis.

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>

strerror_override.h

@@ -0,0 +1,12 @@
+#ifndef __STRERROR_OVERRIDE_H__
+#define __STRERROR_OVERRIDE_H__
+
+#include "config.h"
+#include <errno.h>
+
+#if ENABLE_STRERROR_OVERRIDE
+char *_json_c_strerror(int errno_in);
+#define strerror	_json_c_strerror
+#endif
+
+#endif /* __STRERROR_OVERRIDE_H__ */