run_command() was only run at configure time, meaning the generated
version (that was passed on to the sources via -DYAMBAR_VERSION)
became stale.
Fix by implementing a shell script that generates a header file, and
wrap this in a custom target that is run every time (but the generated
file is only updated when the version changes)
Export project version with -DF00BAR_VERSION.
This is by default set to the meson project version.
However, if git is found, and we manage to run 'git describe' and 'git
branch' in the source directory, version is set to the output of that
instead.
