meson: also allow setting GIT_VERSION via templates

meson: also allow setting GIT_VERSION via templates

GIT_VERSION is not available as a config.h variable, because it's rendered
into version.h during builds. Let's rework jinja2 rendering to also
parse version.h. No functional change, the new variable is so far unused.

I guess this will make partial rebuilds a bit slower, but it's useful
to be able to use the full version string.