fall back to environment variables when building without git

[slanning]

In our RPM build process, for whatever reason
it builds packages from a .git-less source.
This leads to the buildGit* variables not being set
and the corresponding BuildGit* in /debug/vars being empty.
We can export BUILD_GIT_* environment variables
during the build, though; this would fall back to those
in case the git commands come up empty.

Signed-off-by: Scott Lanning [email protected]

[sjmudd]

It might be worth adding a comment to build_version_flags.sh along the lines of

# Normal builds run directly against the git repo but when packaging (for example with rpms) usually
# a tar ball is used (using git archive of the required commit) which will prevent the git metadata
# being available.  Should this be the case then allow environment variables to be used to source
# this information instead.

as otherwise the reasoning may get lost and searching for this PR requires more effort.

Things have changed a lot since rpm was invented but the desire to have pristine sources as part of the src rpm is good as it ensure reproducibility. It would be possible to make the rpm build scripts clone directly from the git repo, and build locally and some people do that but this does mean you don't have full sources. I think there's an open argument these days about which process is better.