Sourcing release version from git tag is fragile

[wkz]

It has been brought to my attention (thanks @troglobit) that some distributions (notably Debian) will always run autoreconf on all packages.

The current flow only considers two scenarios:

1. Building from git clone. configure must be generated, but .git is available so the version can be found.
2. Building from release tarball. configure is already generated so it does not matter that .git is missing.

But in order to support regeneration of configure from release tarballs, the version info must be manually managed in configure.ac. Perhaps we can at least work out some way of appending git version info if it is available.