Stop using git for the version tag.
The new process is as follows:
1. The `VERSION` file contains the version.
For release, it should contain "1.5.0".
Immediately after the release, it should be changed to "1.6.0-dev".
2. The build type is included in the version string unless it is
"Release". The commit hash is now included into the version string
whenever the `VERSION` file has a suffix (i.e. 1.5.0-dev rather than
just 1.5.0). The full version string for development builds now
looks like this: `1.5.0-dev-Debug-b102dccf8`.
This means we longer need git or `-DVERSION_NUM` to get a sensible version number.
The in-development versions are now always clearly indicated with the
`-dev` suffix no matter how DevilutionX was built.
Gleb Mazovetskiy