This takes everything that was in 'weston' the executable a turns it into
library. A new trivial 'weston' executable is written.

Creating the library will allow future improvements:

- we can link weston plugins against the library, meaning that they no longer
  need unresolved symbols to be allowed during linking

- tests do not have to fork() and exec() 'weston', they can just link to the
  library and call wet_main() after setting things up; this will help with
  using a debugger

install_rpath is set so that we can install the library into weston's module
directory, away from the normal libraries in a system. This is one library we
do not intend for others to use.

The library has no stable ABI and is not versioned.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>