git commit hash version extraction during build does not work when build dir is external

[kostrzewa]

Tagging of the build with branch and git commit hash information is extremely useful. However, AFAIU, the current solution does not work if the build directory is not a subdirectory of the source code directory.

$ make
[...]
fatal: Not a git repository (or any parent up to mount point /p/project)
Stopping at filesystem boundary (GIT_DISCOVERY_ACROSS_FILESYSTEM not set).