[PATCH 2/9] GIT-VERSION-GEN: move default version into a separate file

The Git version for Meson is currently wired up manually. It can thus grow (and alread has grown) stale quite easily, as having multiple sources of truth is never a good idea. This issue is mostly of cosmetic nature as we don't use the project version anywhere, and instead use the GIT-VERSION-GEN script to propagate the correct version into our build. But it is somewhat puzzling when `meson setup` announces to build an old Git release.

There are a couple of alternatives for how to solve this:

  - We can keep the version undefined, but this makes Meson output
    "undefined" for the version, as well.

  - We can adapt GIT-VERSION-GEN so that it knows to print a version to
    stdout with a specific invocation and then use `run_command()`. At
    the point of configuring the project we haven't yet figured out host
    details though, and thus we didn't yet set up the shell environment.
    While not an issue for Unix-based systems, this would be an issue in
    Windows, where the shell typically gets provided via Git for Windows
    and thus requires some special setup.

  - We can pull the default version out of GIT-VERSION-GEN and move it
    into its own file. This likely requires some adjustments for scripts
    that bump the version, but allows Meson to read the version from
    that file trivially.

The last option is a proper solution and quite trivial to implement, and adapting scripts should be a one-time event. Refactor GIT-VERSION-GEN accordingly.

This refactoring also prepares for a planned consolidation of all build scripts into a separate folder. This should reduce clutter and make it easier to find all scripts part of the build process.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
---
 GIT-VERSION     | 1 +
 GIT-VERSION-GEN | 4 ++--
 meson.build     | 2 +-
 3 files changed, 4 insertions(+), 3 deletions(-)