Fixed bugs.

* bootstrap:
  * Argument `latest` specified for `--enable-boost` command line option was
    not transformed into the concrete version number.
  * Symbolic link `$prefix/boost/latest` was not created when
    `$prefix/boost/$latest_version` already existed.
Merge branch 'master' into fix/boost
Merge pull request #118 from Cryolite/fix/boost

Fixed bugs.