Added support of `--without-gdb-check` option.

* bootstrap: Added support of `--with-gdb-check` and `--without-gdb-check`
             options.
* jamroot:
  * Added support of `--with-gdb-check` and `--without-gdb-check` options.
  * Added the definition of `GDB_CHECK` global constant.
* gdb/jamfile: Told the build script to skip GDB's `make check` unless
               `GDB_CHECK` is set to `yes`.