libweston: export weston_config API

Make it official that libweston will export the weston_config API, as requested
in https://gitlab.freedesktop.org/wayland/weston/merge_requests/29

 .

There is no other way third party helper clients could access the API.

The autotools build has been accidentally exporting it all the time, but the
Meson build needed fixing.

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