diff --git a/config2.h.in b/config2.h.in
index b6288e93..0b0bb9b8 100644
--- a/config2.h.in
+++ b/config2.h.in
@@ -919,7 +919,7 @@
 #endif
 /* Enable GNU extensions on systems that have them.  */
 #ifndef _GNU_SOURCE
-# undef _GNU_SOURCE
+# define _GNU_SOURCE
 #endif
 /* Enable threading extensions on Solaris.  */
 #ifndef _POSIX_PTHREAD_SEMANTICS