build: Don't use vcs_vmod_version if not present

Before that, when building from a system without Git some Makefile steps would fail because without it vmodtool.py would not generate the required src/vmod_vcs_version.txt file to be present.

We now check if the file is already present or if git is installed, if either condition is validated then src/vmod_vcs_version.txt will be used, if not it will not be used and a warning will be issued at configure time.