gitweb: Create install target for gitweb in Makefile

gitweb: Create install target for gitweb in Makefile

Installing gitweb is now as easy as

  # make gitwebdir=/var/www/cgi-bin gitweb-install  ;# as root

The gitweb/INSTALL file was updated accordingly, to make use of this
new target.

Fix shell quoting, i.e. setting bindir_SQ etc., in gitweb/Makefile.
Those variables were not used previously.

Signed-off-by: Jakub Narebski <redacted>
Signed-off-by: Junio C Hamano <redacted>