Include json_inttypes.h in json_object.h since we use types like int32_t in the API and some systems need that header to compile.

As part of this create a public json_config.h with a custom define to
decide whether to include inttypes.h to avoid conflicting with other
projects config.h header.

json_object.h

@@ -12,6 +12,8 @@
 #ifndef _json_object_h_
 #define _json_object_h_
 
+#include "json_inttypes.h"
+
 #ifdef __cplusplus
 extern "C" {
 #endif