Fixed #78.

* bootstrap:
  * Modified to dereference `latest` value of `--with-gmp-for-gcc` and
    `--enable-gmp` command-line options into concrete version numbers before
    invoking `bjam`.
  * Added support for irregular version strings of GMP.
* gmp/download.sh: Newly added.
* jamroot: Removed support for `latest` value of `--with-gmp-for-gcc` and
  `--enable-gmp` command-line options.
* gmp/jamfile:
  * Removed downloading and expanding actions.
  * Switched dependency on GMP source trees from `gmp-$VERSION/README`
    to timestamp files
    `gmp-${VERSION}/2bf4ddbc-6fd0-44c5-82ce-d66a6eb45809`.
* gcc/jamfile: Switched dependency on GMP source trees, likewise as above.