Building outside a git repo

[aidanheerdegen]

libaccessom2 has to build inside a git repository to create a git hash for versioning

libaccessom2/CMakeLists.txt

Lines 120 to 125 in d750b4b

	execute_process(
	COMMAND git rev-parse HEAD
	WORKING_DIRECTORY ${CMAKE_SOURCE_DIR}
	OUTPUT_VARIABLE GIT_COMMIT_HASH
	OUTPUT_STRIP_TRAILING_WHITESPACE
	)

This is the same for MOM5 mom-ocean/MOM5#308, which this PR is changing mom-ocean/MOM5#384

For the same reasons as MOM5 it would be good to have the option of build libaccessom2 without needing to be in a git repo.

[aidanheerdegen]

Tested this, and it does build ok outside a git repo, but it will emit an error from git and then set GIT_COMMIT_HASH to an empty string.

Best to fix anyway to make behaviour explicit and not emit errors which can be confusing.