Add Git version string support.

Introduce a script to auto-generate version strings. Use `git describe`
to create descriptive version strings when building from a Git checkout,
use ".version" files for release tarballs.

Signed-off-by: Lukas Fleischer <calcurse@cryptocrack.de>

.gitignore

@@ -11,6 +11,7 @@ depcomp
 install-sh
 missing
 stamp-h1
+.version
 autom4te.cache/
 m4/