Change the strerror_override handling to check $_JSON_C_STRERROR_OVERRIDE instead of using a variable, so we don't need to export it.

tests/test_util_file.c

@@ -127,8 +127,6 @@ int main(int argc, char **argv)
 	//	json_object_to_file(file, obj);
 	//	json_object_to_file_ext(file, obj, flags);
 
-	_json_c_strerror_enable = 1;
-
 	const char *testdir;
 	if (argc < 2)
 	{