Migrate to dynamic VERSION_CURRENT

The application version is determined at build time by looking in VERSION_CURRENT. That file is maintained manually. In an effort to move toward a more automated process, this file should be generated by a script (using the same approach described in CXX-584).

A. Jesse Jiryu Davis, Kevin Albertson, as part of this I would like to eliminate VERSION_RELEASED entirely. The only use of it I found was in assembling the URL for GitHub tarball download. Is there an alternative approach that would allow eliminating it?