Describe the bug
When tagging a beta release, the built-in version string will be the standard non-release string.

To Reproduce
Tag a beta release such as r3_7_0beta3.

Expected behavior
The version should be 3.7.0beta3 (haven't found any historic beta tags, but this syntax would be consistent with commit-based build numbers (3.6.2git-XXX)

Screenshots
The default non-release logic kicks in:

$ git checkout r3_7_0beta3
$ qmake Jamulus.pro 
Project MESSAGE: building version "3.6.2dev-6e630f49" (intermediate in git repository)

@nefarius2001 Could you tackle that?

cc @ann0see @pljones