Make use of crate config to get version

[mgrojo]

This allows changing the version only in the alire.toml file.

[SMerrony]

Although I appreciate the DRY aspect of this PR, it feels "inside-out" to me...
I don't want the application source to depend on the build system (which could change).
It would make more sense to me if Alire could pick up the version from the application source.

[mgrojo]

This is the solution proposed by Alire, so you have a centralized definition of the software version. What they also recommend, it's adding the generated config/ files to git, so the project can still be built without Alire, provided that the user adds the dependencies themselves and set GPR_PROJECT_PATH accordingly. I should have done also that part, but, in any case, I don't know if that would address your concern.

In any case, you're free to disregard this pull request, of course. I just couldn't resist proposing it after seeing your comment in the version constant 😃

Regarding Alire picking up the version from application source, I think it would be more cumbersome than the current approach. They have a general mechanism for crate configuration which also solves the problem of centralized version, but it's useful for other issues.