Makefile: Fix 'clean' target to remove all gitweb build files

In particular the gitweb/GITWEB-BUILD-OPTIONS file was not being
removed by the main Makefile. However, the gitweb/Makefile has a
'clean' target that correctly removes all the build products.
In order to fix the problem, rather than duplicate the clean-up
instructions, we change the main Makefile so that it delegates
the clean-up actions to the gitweb Makefile.

Signed-off-by: Ramsay Jones <ramsay@ramsay1.demon.co.uk>
Acked-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Makefile

@@ -2098,7 +2098,7 @@ clean:
 	$(RM) $(htmldocs).tar.gz $(manpages).tar.gz
 	$(MAKE) -C Documentation/ clean
 ifndef NO_PERL
-	$(RM) gitweb/gitweb.cgi gitweb/gitweb.min.*
+	$(MAKE) -C gitweb clean
 	$(MAKE) -C perl clean
 endif
 ifndef NO_PYTHON