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>

Makefile

@@ -272,6 +272,7 @@ mandir = share/man
 infodir = share/info
 gitexecdir = libexec/git-core
 sharedir = $(prefix)/share
+gitwebdir = $(sharedir)/gitweb
 template_dir = share/git-core/templates
 htmldir = share/doc/git-doc
 ifeq ($(prefix),/usr)