CMake script to check if PLT file is up-to-date

With the ErlangCheckPlt it is possible to check whether a dialyzer
generated PLT file is up-to-date or not. If it is not, it is deleted.

Change-Id: I1194c8db44a662821e8f55d435ee2e34d4c61707
Reviewed-on: http://review.couchbase.org/41813
Reviewed-by: Sarath Lakshman <sarath@couchbase.com>
Reviewed-by: Chris Hillery <ceej@couchbase.com>
Tested-by: Volker Mische <volker.mische@gmail.com>