Guidebook make sequencing

Be able to run 'make Guidebook{|.txt|.ps}' in the doc subdirectory
without requring a full install first.

makedefs is no longer one of the first things built when building
nethack, and even if it was, nobody should have to rebuild all of
nethack after 'make spotless' when they just want to create one
or more of the formatted Guidebooks.  The 'roff versions (.ps and
also .txt) use makedefs as a filter by default, so wouldn't build
if it wasn't there.

Have Makefile.doc build makedefs when needed.

Clean up some dependencies in Makefile.utl.

Add a couple of comments to Makefile.src.

sys/unix/Makefile.src

@@ -710,7 +710,8 @@ $(MAKEDEFS): $(FIRSTOBJ) \
 		../include/patchlevel.h
 	@( cd ../util ; $(MAKE) makedefs )
 
-#	Source files created by 'makedefs' at build time.
+#	Source files formerly created by 'makedefs' at build time
+#	but no longer used.
 #	Each is given an artificial dependency upon the one before
 #	so that parallel makes will have to build them sequentially.
 #	(More for documentation than effect; 'make' should know not
@@ -771,7 +772,8 @@ clean:
 spotless: clean
 	-rm -f a.out core $(GAMEBIN) Sys*
 	-rm -f ../include/nhlua.h
-	-rm -f ../include/date.h ../include/onames.h ../include/pm.h
+	-rm -f ../include/date.h #created but no longer used, at least by core
+	-rm -f ../include/onames.h ../include/pm.h #obsolete generated files
 	-rm -f tile.c *.moc
 	-rm -f ../win/gnome/gn_rip.h