Improve Makefile generation of version files.

Remove bad automake dependency causing double execution.
Properly update the version in the non-Git case.
Git commands are now in a condition-expression, won't break build.
Test that we're in repo root (i.e., our own and not somebody else's).
Do `git describe --always` to handle shallow Git clones.

Signed-off-by: John Hood <cgull@glup.org>