Create a sysconfdir variable, and use it for ETC_GITCONFIG

Create a sysconfdir variable, and use it for ETC_GITCONFIG

ETC_GITCONFIG defaults to $(prefix)/etc/gitconfig, so if you just set
prefix=/usr, you end up with a git that looks in /usr/etc/gitconfig, rather
than /etc/gitconfig as specified by the FHS.  Furthermore, setting
ETC_GITCONFIG does not fix the paths to any future system-wide configuration
files.

Factor out the path to the system-wide configuration directory into a variable
sysconfdir, normally set to $(prefix)/etc, but set to /etc when prefix=/usr .
This fixes the prefix=/usr problem for ETC_GITCONFIG, and allows centralized
configuration of any future system-wide configuration files without requiring
further action from package maintainers or other people building and
installing git.

Signed-off-by: Josh Triplett <josh@freedesktop.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>