diff --git a/tools/m4/ratbox.m4 b/tools/m4/ratbox.m4
index 2977f4efa..082efc760 100644
--- a/tools/m4/ratbox.m4
+++ b/tools/m4/ratbox.m4
@@ -28,3 +28,5 @@ AC_DEFUN([CPPDEFINE],
 ])
 
 AC_DEFUN([RB_HELP_STRING], [AS_HELP_STRING([$1], [$2], 40, 200)])
+
+AC_DEFUN([RB_VAR_PREPEND], [AS_VAR_SET([$1], ["$2 ${$1}"])])