git: install git l10n using --with-gettext option

Jack Nagel suggests gettext option should disable by default, and
it is unnecessary to provide CFALGS/CPPFLAGS and LDFLAGS. Rework
on patch 444d559 (git: add `without-gettext` option), and update
install instuctions for git in README.md.

Signed-off-by: Jiang Xin <worldhello.net@gmail.com>