The Git configuration value ocaml.configure is now passed to the
configure script's arguments before $@ if (and only if) OCaml is being
configured from a Git clone.
This allows, for example:
- Developer-specific preferences (e.g. `--disable-ocamldoc` or
`--disable-debug-runtime`)
- Automatic use of autoconf cach files (-C option)
It is implemented by inserting a test at the top of `configure`, which
is bypassed if `.git` doesn't exist.