Build: Make dependency on git change more robust

Build: Make dependency on git change more robust

It can sometimes happen that .git/HEAD or .git/refs/head/*, which
are added as config.status dependencies during configure, do not
exist anymore at build time.  For instance, when the current branch
is deleted after switching to a different one.

Wrap the dependencies inside $(wildcard ...) to avoid this problem.
Note that this is a GNU make feature.  However, it should be fine
as it is only used for git builds.  Even if a non-GNU make is used,
the construct will hopefully just expand to nothing.