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>