Added support of GDB.

* .gitignore: Told to ignore `gdb-*` directory.
* bootstrap: Added support of `--enable-gdb`.
* jamroot:
  * Added handling of `--enable-gdb`.
  * Added `<gdb>` and `<gdb-hidden>` features.
* gdb/jamfile: New. Added build script.
* gdb/latest.sh: New. Added script to extract the latest version of GDB.