Switched to setting up the GaudiKernel target as an alias, while
making sure that the GaudiKernel headers would be treated as system
headers.

Found that Gaudi finding CLHEPConfig.cmake internally was causing
some nasty effects on AtlasLCG's ability to use its own
FindCLHEP.cmake code. Resulting in some really weird hysteresis
behaviour in what CMake would do when configuring an ATLAS project.
This is fixed by forefully removing the variables at the end of
FindGaudi.cmake that make FindCLHEP.cmake confused.