diff --git a/config.h.cmake.in b/config.h.cmake.in
index a165424..91dff67 100644
--- a/config.h.cmake.in
+++ b/config.h.cmake.in
@@ -14,6 +14,7 @@
 #cmakedefine HAVE_FLTK_DEAD_KEYS
 #cmakedefine HAVE_FLTK_CLIPBOARD
 #cmakedefine HAVE_FLTK_MEDIAKEYS
+#cmakedefine HAVE_FLTK_FULLSCREEN
 #cmakedefine ENABLE_NLS 1
 
 /* MS Visual Studio 2008 and newer doesn't know ssize_t */