Added support of `--with-boost-check` option.

* bootstrap: Added support of `--with-boost-check` and
  `--without-boost-check` options.
* jamroot:
  * Added support of `--with-boost-check` and `--without-boost-check`
    options.
  * Added the definition of `BOOST_CHECK` global constant.
* boost/jamfile: Tells the build script to skip Boost unit tests when
  `BOOST_CHECK` is set as other than `yes`.