diff --git a/src/libmdb/fakeglib.c b/src/libmdb/fakeglib.c
index f0f51f8..d980cf9 100644
--- a/src/libmdb/fakeglib.c
+++ b/src/libmdb/fakeglib.c
@@ -1,6 +1,7 @@
 
 #include "mdbfakeglib.h"
 
+#define _GNU_SOURCE /* vasprintf */
 #include <stddef.h>
 #include <stdlib.h>
 #include <stdarg.h>