weston-wfits: use our own Weston SDK version macro

The WESTON_VERSION_AT_LEAST macro logic does not do what one
would expect if taking its name literally.  Upstream concensus
is divided about what it should be (i.e. naming vs. logic).

Therefore, we're not even going to use it... we'll define our
own means for supporting multiple Weston SDK versions including
unstable/development branches.

Signed-off-by: U. Artie Eoff <ullysses.a.eoff@intel.com>