Fixed #86

* .gitignore: Told to ignore downloaded GDB tarballs.
* bootstrap:
  * Modified to dereference `latest` value of `--enable-gdb` command-line
    option into concrete version numbers before invoking `bjam`.
* gdb/download.sh: Newly added.
* jamroot: Removed support for `latest` value of `--enable-gdb` command-line
  option.
* gdb/jamfile:
  * Removed downloading and expanding actions.
  * Switched dependency on GDB source trees from `gdb-$VERSION/README`
    to timestamp files `gdb-$VERSION/f4f591f6-9a1d-4ac3-ae2f-ba6463d79680`.