Include version number in build

pkg-config has a version number field that was being filled out as "?"
but is now actually being populated.  Additionally, there is a bit of
autodetection here.  The rules are

* If $srcdir is a git repository, then `git describe` produces the
  version number (with a "v" removed from the front of the tag name).

* If $srcdir isn't a git repository, then a hard-coded version string
  is used from the configure.ac file.

* Whenever configure is run and $srcdir is a git repository, the
  hard-coded version string is checked against the tag.

This is a modification of the versioning mechanism I use for my own
projects adapted to work with autoconf.  Note that it's not well
tested (as I don't really know any autoconf magic), but it does seem
to work for me.

Also: I ran "autoconf" and checked in the result.  I seem to have a
newer autoconf than whomever ran it last, hopefully that's OK.