[CMake] Allos use of CPack

Including of CPack has to be done after defining all the installation
targets.
Haiku: export API classes

The CMake build system makes the classes hidden by default. Explicitly
export everything that's part of the API so it's possible to link to it.
Haiku: use CPack to build HPKG packages

Unfortunately, CMake requires things to happen in the right order, which
makes it difficult to do this without changes to the generic build
files.

We now build two hpkg files, one with development stuff and another with
the libWebKit shared library.

To build working packages, don't forget to set the cpack maintainer when
confguring the build. Use -DCPACK_PACKAGE_CONTACT="your email address".
If you don't do that, CPack won't let you generate a package.

To use the CPACK generator, you need CMake 2.8.11.2-3 or later.