make uncompiled exported headers compilable
(shamelessly stolen inspired by https://gitlab.cern.ch/clemenci/Gaudi/commits/GAUDI-1231.add-test-build-of-all-headers)
The idea is that any public header "GaudiComponent/UseMe.h" should be usable in a oneline "UseMe.cpp" that includes the header and nothing else. (i.e. anding an include of a Gaudi header should not change a file from compilable to non-compilable).