build git without i18n if gettext is not available
also fix the install target for git without gettext
Trac #15401: Build git without gettext if necessary, spkg-check sanity

The git spkg fails to build if gettext is not installed, see
https://groups.google.com/d/msg/sage-devel/RAWInaDhz5c/lQYXrKQ4aXEJ

We should try again without gettext if necessary.

Also, clean up spkg-install/spkg-check.

URL: http://trac.sagemath.org/15401
Reported by: vbraun
Ticket author(s): Volker Braun
Reviewer(s):