git-version-gen: Add default version
git-version-gen: Add default version

Instead of bailing out early in the build process when neither a version
file nor a valid Git repository is detected, use a hardcoded default
version string. This fixes building from shallow clones or Git
snapshots.

Also fixes GitHub issue #22.

Signed-off-by: Lukas Fleischer <c9ca73f8336dcfb037ec2c2abf52d80a6847a5cd@calcurse.org>

lfos/calcurse,lfos/calcurse,lfos/calcurse