Compute the version number dynamically

Compute the version number dynamically

This creates a generated version number. When not on the master
branch (i.e., a release branch) it uses git describe --tags to return
either the exact release tag or if on a later commit (because there
was a fix made to a released version) then the command returns that
tag plus the number of commits since that tag along with the short
hash. When on the master branch, which has no tags, it returns a
similiar output.

Signed-off-by: Jason Self <j@jxself.org>