Merge "Fix the JSON extension build stuff" into 3.1

Merge commit c58153cca4f5284d3a44649b876957eb4800eee9 into 3.1 branch.
Without this fix fatal error in jsonc_parser.cpp has been thrown when compiling HHVM due to implicit use of json-c on systems without this library:
```
hphp/runtime/ext/json/jsonc_parser.cpp:28:23: fatal error: json/json.h: No such file or directory
 #include <json/json.h>
                       ^
compilation terminated.
make[2]: *** [hphp/runtime/ext/CMakeFiles/hphp_runtime_ext.dir/json/jsonc_parser.cpp.o] Error 1
make[1]: *** [hphp/runtime/ext/CMakeFiles/hphp_runtime_ext.dir/all] Error 2
make: *** [all] Error 2
```