We want to embed the current commit-id in the ./configure script.
That way the generated ./configure file in the source tarball
references the commit-id from which the tarball was created.
Then, in a second step, a script can check ./configure to find
the parent commit. This is for example done by the 'makerepo.sh'
script.
This is generally useful, and also done by network-manager-applet
and libnl3 projects. Move the function to a separate m4 macro
to reuse it. It should also be re-used in NetworkManager's VPN plugins.
