configure: Maintainer mode requires imagemagick.
diff --git a/Make.rules.in b/Make.rules.in
index f658412..654dbc3 100644
--- a/Make.rules.in
+++ b/Make.rules.in
@@ -48,6 +48,7 @@
 LINTFLAGS = @LINTFLAGS@
 FONTFORGE = @FONTFORGE@
 RSVG      = @RSVG@
+CONVERT   = @CONVERT@
 ICOTOOL   = @ICOTOOL@
 CROSSCC   = @CROSSCC@
 CROSSTARGET = @CROSSTARGET@