diff --git a/config.h.cmake b/config.h.cmake
index 4a31452a..4978aeaf 100644
--- a/config.h.cmake
+++ b/config.h.cmake
@@ -1,4 +1,4 @@
 #cmakedefine PACKAGE_NAME "@PACKAGE_NAME@"
 #cmakedefine PACKAGE_VERSION "@PACKAGE_VERSION@"
 #cmakedefine PACKAGE_STRING "@PACKAGE_STRING@"
-#cmakedefine _XOPEN_SOURCE "@_XOPEN_SOURCE@"
+#cmakedefine _XOPEN_SOURCE @_XOPEN_SOURCE@