diff --git a/mem.h b/mem.h
index 980ea13..af89ba6 100644
--- a/mem.h
+++ b/mem.h
@@ -41,7 +41,9 @@ extern "C" {
 #ifdef HAVE_ALLOCA_H
 # include <alloca.h>
 #elif defined __GNUC__
+#if !defined alloca
 # define alloca __builtin_alloca
+#endif
 #elif defined _AIX
 # define alloca __alloca
 #elif defined _MSC_VER