configure.ml: our configure script is now written in ML :-)

  configure is now just a minimal wrapper around the new configure.ml.
  This configure.ml is runned with the same ocaml used during
  compilation, and starts with a #load "unix.cma".
  For now, this new configure script is meant to be 99% compatible
  with the old one. Known incompatibilities : the --foo option format
  (with two --) isn't supported anymore, use -foo options instead.
  Let me know if you encounter any other changes.

  Internals:
   - We use our own "run" command (based on Unix.create_process) to avoid
     relying on some specific shell (/bin/sh or cmd.exe).
   - We should have far less issues with filename quoting under windows
     since we almost never rely on (cygwin) shell anymore. This remains
     to be fully tested, though.
   - dev/ocamldebug-coq is slightly different now, to ease its generation

Author: letouzey