Made the logic in GaudiProjectConfig.cmake compatible with the GAUDI_ATLAS=TRUE mode.

This required delaying the finding of Python and Boost until the call to gaudi_project(...),
to let the base ATLAS project set everything up for finding these things correctly.

At the same time updated the default ATLAS project name to AthenaExternals. Since that
has been our offline software's external project for some time now.