Prevent compilation with empty version

Compiling the project without git installed uses a hardcoded
version with a *-dev ending in the dev versions instead of
the specific commit information after the version.

docs/dev/release.md

@@ -4,3 +4,4 @@
  - Update travis version
  - Releases always use annotated tags as in `git tag -a v0.5.1 -m "version 0.5.1"`
  - Add a changelog description in the Github's release.
+ - Update .pro hardcoded version.