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>

random_seed.c

@@ -9,6 +9,7 @@
  *
  */
 
+#include "strerror_override.h"
 #include <stdio.h>
 #include "config.h"
 #include "random_seed.h"
@@ -128,7 +129,6 @@ retry:
 #include <string.h>
 #include <fcntl.h>
 #include <unistd.h>
-#include <errno.h>
 #include <stdlib.h>
 #include <sys/stat.h>