diff --git a/src/xlstool.c b/src/xlstool.c
index 54eab0e..ef1c7e9 100644
--- a/src/xlstool.c
+++ b/src/xlstool.c
@@ -32,2 +32,3 @@
 
+#define _GNU_SOURCE /* asprintf */
 #include "config.h"