Instead of trying to run `git --rev-parse --is-inside-work-tree`,
check if there's a .git directory under the source directory.
This should fix an issue where we incorrectly decided we where in a
foot git clone when we're just a subdirectory under another
repository.
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)
