git-gui: replace GIT-GUI-VARS with GIT-GUI-BUILD-OPTIONS

git-gui: replace GIT-GUI-VARS with GIT-GUI-BUILD-OPTIONS

The GIT-GUI-VARS file is used to track whether any of our build options
has changed. Unfortunately, the format of that file does not allow us to
propagate those build options to other scripts. But as we are about to
introduce support for the Meson build system, we will extract a couple
of scripts to deduplicate core build logic across Makefiles and Meson.
With this refactoring, it will become necessary to make build options
more widely accessible.

Replace GIT-GUI-VARS with a new GIT-GUI-BUILD-OPTIONS file that is being
populated from a template. This file can easily be sourced from build
scripts in subsequent steps.

Signed-off-by: Patrick Steinhardt <ps@pks.im>