Change the package version to 0.1.999 to distinguish the master branch from
any release versions.

Release versions will get their own branches and tags with a specific version
number.

This is likely a temporary arrangement. For the near future, any releases will
likely be taken from specific commits in master that have gotten adequate
testing. This provides an easy way to tag those releases.

Using a large revision number for master ensures that other autoconf (or at
least pkg-config based) builds that expect a minimum version number will also
work with the latest master branch.