[PATCH v6 08/15] Makefile + test-lib.sh: don't prefer cmake-built to make-built git

Per ee9e66e4e76 (cmake: avoid editing t/test-lib.sh, 2022-10-18) and the surrounding discussion the goal of that change was to have building with cmake and:

	(cd t && ./t0001-init.sh)

Work, to do so we need some way to make the test-lib.sh aware that we have a build in contrib/buildsystems/out. The downside of doing it via the GIT-BUILD-DIR facility in ee9e66e4e76 is:

- A "remove this file while at it" only fits into the Makefile
  dependency graph if we "FORCE" a shell command to be run, so doing it
  this way precludes speeding up the Makefile by e.g. making use of the
  "$(file)" function (for which I've got follow-up patches).

- If you build with make, then with cmake, and then e.g.:

	make -C t prove

  We'll run the tests against the cmake version of the tests, not the
  "make" built ones. It's important to get any subtle interaction
  between two conflicting build systems right.

Now we'll instead check if the top-level "git" exists, and only if it doesn't look for the one built by cmake. This ensures that we'll pick it up as required when the user is using CMake, without being subject to the odd interaction of flip-flopping between the two environments.

It also means that we can remove the rule to remove the "GIT-BUILD-DIR", if we get to creating "GIT-BUILD-OPTIONS" in the top-level git. That's because now when building with both "make" and "cmake" we will always prefer the former for running the tests, so there's no need to remove the "GIT-BUILD-DIR" file to resolve the conflicts between the two.

In practice users are building with either "make" or "cmake", and not mixing the two, but in a subsequent change we'll provide an out for that "make"-preferring selection behavior, by having "ctest" runs always prefer the "cmake" built one.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
---
 Makefile      | 1 -
 t/test-lib.sh | 3 ++-
 2 files changed, 2 insertions(+), 2 deletions(-)