--- a/m4macros/gtkglextmm.m4
+++ b/m4macros/gtkglextmm.m4
@@ -222,7 +222,7 @@
 dnl AC_GTKGLEXTMM_SUPPORTS_MULTIHEAD([ACTION-IF-SUPPORTED [, ACTION-IF-NOT-SUPPORTED]])
 dnl Checks whether gtkglextmm supports multihead.
 dnl
-AC_DEFUN(AC_GTKGLEXTMM_SUPPORTS_MULTIHEAD,
+AC_DEFUN([AC_GTKGLEXTMM_SUPPORTS_MULTIHEAD],
 [ AC_LANG_SAVE
   AC_LANG_CPLUSPLUS
   AC_CACHE_CHECK([whether gtkglextmm supports multihead],