Use CC for fuzzer, not CXX

test/fuzz.c

@@ -88,12 +88,16 @@ int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
 
 #if defined(MAIN)
 int main(int argc, char *argv[]) {
+  int res = EXIT_FAILURE;
   if (argc > 1) {
     size_t len = 0;
     char *buf = mg_file_read(&mg_fs_posix, argv[1], &len);
-    if (buf != NULL) LLVMFuzzerTestOneInput((uint8_t *) buf, len);
+    if (buf != NULL) {
+      LLVMFuzzerTestOneInput((uint8_t *) buf, len);
+      res = EXIT_SUCCESS;
+    }
     free(buf);
   }
-  return 0;
+  return res;
 }
 #endif