configure: silent complaint if not built on git repository
This silences a complaint if the toolkit is not built within a Git
repository:
...
[*] Checking libnet ... [YES]
fatal: Not a git repository (or any of the parent directories): .git
[*] Generating config.h ...
[*] Looks good! All tools will be built!
This is not problematic as the build can be done without any problems.
It will just have an empty Git id in --version output. So simply
silence this complaint and move on.
Signed-off-by: Daniel Borkmann <dborkman@redhat.com>