On the revision:

- Make the .map and .def files prerequisites of the libs type target and only
  on the respective OSes.

- Fix the switch statements in the buildfiles, replacing 'switch $c.class,
  $tsys' with 'switch $c.class'.

- Canonicalize the license in the package manifest.

- Disable the success build emails.

- Make libz/LICENSE a symlink to the file in the project root directory.

On the release:

- Use legal{} target type for legal documentation (LICENSE, AUTHORS, etc).

  Note that this requires bumping build2 version requirement to 0.13.0.