Previously, our secondary DA response indicated a) VT420, b) an XTerm
version number.
Now, we indicate VT220 (which corresponds to the primary DA response),
and we report foot's version number as MMmmpp. I.e major, minor and
patch versions, using two digits.
E.g. 1.4.2 is encoded as 010402
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 -DFUZZEL_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)
