AC_INIT requires that the version string passed in be 'static'
so we can't use the output of a shell invocation. However, it
can be computed by an m4 shell invocation. This is what autoconf
itself does to embed its git version number in PACKAGE_VERSION.

Doing this necessitates the 'echo -n' in git-version.sh. Make
is smart enough to strip the trailing newline, but AC_INIT is
not. We must also add -f to autoreconf. Otherwise, autogen.sh
will only update configure when configure.ac changes, even if
the output of git-version.sh has changed.

This is still not ideal, since it runs at autogen.sh time, not
at 'make' time, which is what we want. This seems to be a limitation
of an upcoming automake release, which removes the old style
with PACKAGE and VERSION set by shell variables.