After talking to @jow on IRC, we found two problems. STAGING_DIR_HOST is
designed for tools, not host packages. Changing this to HOSTPKG allows
CMake and pkgconfig to work properly with fbthrift.

The host-libraries should not be modular like this for host packages.
Changed to eliminate them and to only build the needed ones. This can be
changed as conditions change.

Signed-off-by: Rosen Penev <rosenp@gmail.com>