Optional git env variables

[rapiz1]

Follows up #92

Git repository is not always present along with the source code, e.g. downloading source tarball. If feature git is enabled and no git repository is present, the build script using vergen will fail to run and abort the compilation. I think a better way to handle the situation with no available repository is that simply don't set corresponding Git env vars, instead of failing completely. Then by option_env!, the user can determine a fallback method.

[rapiz1]

My fault. Just don't return the Err that vergen() returns, then the build script won't fail :)