Rename labels.

Rename labels from FOO to label_foo in order to avoid system macro
definitions, in particular OUT and ERROR on mingw.

Reported by Mike Hommey.

src/chunk_dss.c

@@ -293,11 +293,11 @@ chunk_dealloc_dss(void *chunk, size_t size)
 			madvise(chunk, size, MADV_DONTNEED);
 
 		ret = false;
-		goto RETURN;
+		goto label_return;
 	}
 
 	ret = true;
-RETURN:
+label_return:
 	malloc_mutex_unlock(&dss_mtx);
 	return (ret);
 }