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>

tests/test_json_pointer.c

@@ -1,4 +1,4 @@
-#include <errno.h>
+#include "strerror_override.h"
 #include <assert.h>
 #include <stdio.h>
 #include <string.h>