build: add option to build static libweston library

So far libweston comes in shared library form and it is available in many distros. However, different distros has vastly different libweston verson and use different options. Compositors other than weston itself would have hard time adapting to different distros and keep up with faster iteration of libweston. Having libweston to build as static library allows other compositor to link against different libweston versions and not conflict with system libweston.

Signed-off-by: xichen sichem.zh@gmail.com