gitweb: Set default destination directory for installing gitweb in Makefile

gitweb: Set default destination directory for installing gitweb in Makefile

Currently installing gitweb requires to give a target directory
(via 'gitwebdir' build variable).  Giving it a default value
protects against user errors.

Signed-off-by: Pavan Kumar Sunkara <pavan.sss1991@gmail.com>
Mentored-by: Christian Couder <chriscool@tuxfamily.org>
Mentored-by: Petr Baudis <pasky@ucw.cz>
Acked-by: Jakub Narebski <jnareb@gmail.com>
Acked-by: Petr Baudis <pasky@suse.cz>
Signed-off-by: Junio C Hamano <gitster@pobox.com>