Detect the snapshot version number using darcs

Detect the snapshot version number using darcs
For non-release builds, we want to append a date to the version number
(e.g. 6.7.20070206).  Previously this was done by the nightly build
script, this new method figures out the snapshot version by querying
the darcs repository and finding the date of the most recent patch
(actually it finds the most recent of the last 100 patches, but that
should be good enough).  This is done by the configure script.

To handle source distributions, we create a file VERSION in the
top-level directory that contains the version number, and ship this in
the source distribution.  The configure script picks up the version
from this file if it doesn't see a _darcs directory.