Makefile: use common template for GIT-BUILD-OPTIONS

The "GIT-BUILD-OPTIONS" file is generated by our build systems to
propagate built-in features and paths to our tests. The generation is
done ad-hoc, where both our Makefile and the CMake build instructions
simply echo a bunch of strings into the file. This makes it very hard to
figure out what variables are expected to exist and what format they
have, and the written variables can easily get out of sync between build
systems.

Introduce a new "GIT-BUILD-OPTIONS.in" template to address this issue.
This has multiple advantages:

  - It demonstrates which built options exist in the first place.

  - It can serve as a spot to document the build options.

  - Some build systems complain when not all variables could be
    substituted, alerting us of mismatches. Others don't, but if we
    forgot to substitute such variables we now have a bogus string that
    will likely cause our tests to fail, if they have any meaning in the
    first place.

Backfill values that we didn't yet set in our CMake build instructions.
While at it, remove the `SUPPORTS_SIMPLE_IPC` variable that we only set
up in CMake as it isn't used anywhere.

This change requires us to adapt the setup of TEST_OUTPUT_DIRECTORY in
"test-lib.sh" such that it does not get overwritten after sourcing when
it has been set up via the environment. This is the only instance I
could find where we rely on ordering on variables.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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