The Frontier client (libfrontier_client.so) was brought in by the now
deleted `TrigConfDBConnection` package. Add this dependency explicitly
to the Athena project (as is already the case for AthGeneration).

The hash used to decide if the externals have to be rebuild only
considered the first entry of `EXTRACMAKE`. All others (e.g. the new
Gaudi version argument) were ignored.

These are necessary for the basic operation of the project.