Fixed #69.

* bootstrap: Changed to dereference `latest` value of `--with-cloog-for-gcc`
  and `--enable-cloog` command-line options into concrete version numbers
  before invoking `bjam`.
* binutils/download.sh: Tweaked.
* binutils/checkout.sh: Tweaked.
* cloog/download.sh: Newly added.
* cloog/checkout.sh: Newly added.
* jamroot: Removed support for `latest` value of `--with-cloog-for-gcc` and
  `--enable-cloog` command-line options.
* cloog/jamfile:
  * Removed downloading and expanding actions.
  * Switched dependency on CLooG source trees from `cloog-$VERSION/README`
    to timestamp files
    `cloog-${VERSION}/528457b5-9e38-4c8f-ac97-fbdb3ccfdbe9`.
* gcc/jamfile: Switched dependency on CLooG source trees, likewise as above.