Properly persist git hash on each build
Rebuild `git_hash.h` only when `.git` folder changes
`add_custom_target()` always considers its target out-of-date which
leads to rebuilding of `git_hash.h` on every build (which is
ironically what we wanted) and consequently rebuilding of
build_options.c and relinking of c3c even when no changes are made,
which is mildly annoying.
We are replacing `add_custom_target()` with `add_custom_command()`
which depends on `.git`, so `git_hash.h` is only rebuilt if any git
commands are performed. Which is less annoying.
In case of no `.git` we simply do not depend on it which leads to
`git_hash.h` being rebuilt only once.
