diff --git a/src/psl.c b/src/psl.c
index 85e9859..15fbb6d 100644
--- a/src/psl.c
+++ b/src/psl.c
@@ -24,7 +24,10 @@
  *
  */
 
-#define _GNU_SOURCE
+// need _GNU_SOURCE for qsort_r()
+#ifndef _GNU_SOURCE
+#	define _GNU_SOURCE
+#endif
 
 #if HAVE_CONFIG_H
 # include <config.h>