Attached, a patch to improve valac version detection. The vala compiler,
valac, can report the API version it supports. For releases, this is the
same as the major & minor version of the compiler, so for example:

$ valac --version
Vala 0.48.11
$ valac --api-version
0.48

The advantage of using the API version is that it's semantically
meaningful. It's also simpler to parse! It can be to allow a 3rd-party,
patched, or unreleased compiler to be used more easily.

(For example, the current Vala git master compiler reports:

$ valac-0.52 --version
Vala 0.50.1.19-135c0
$ valac-0.52 --api-version
0.52

)

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