Disable calling a target which wants ImageMagick.

Index: doc/Makefile
--- doc/Makefile.orig
+++ doc/Makefile
@@ -46,7 +46,7 @@ giflib-logo.gif: ../pic/gifgrid.gif
 	convert $^ -resize 50x50 $@
 
 # Philosophical choice: the website gets the internal manual pages
-allhtml: $(XMLALL:.xml=.html) giflib-logo.gif
+allhtml: $(XMLALL:.xml=.html)
 
 manpages: $(XMLMAN1:.xml=.1) $(XMLMAN7:.xml=.7) $(XMLINTERNAL:.xml=.1)
 
