build: Automatically parse version from tag

The build will now parse tags so the git tag is now the single source
of truth for a version. We do not need to manually maintain the
version header.

The VERSION file uses git's export-subst feature. When git archive
creates a snapshot (e.g. cgit), git substitutes the $Format:...$
placeholder with the output of git describe, embedding the version
string directly in the archive. This requires the export-subst
attribute to be set for the VERSION file in the repository's
.gitattributes.

The version is resolved in the following order:
 - git describe (inside a git repository with tags)
 - the VERSION file (archives/snapshots)
 - 0.0.0 (fallback)

The build system will warn when it can't set a correct version, such
as in a git repo without tags, or an archive without VERSION.

Signed-off-by: Dimitri Staessens <dimitri@ouroboros.rocks>
Signed-off-by: Sander Vrijders <sander@ouroboros.rocks>

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