Warnings hidden from upstream project headers

For example, when compiling in Rec, LHCb headers are found as system headers:

... -isystem /home/rmatev/stack/LHCb/InstallArea/x86_64_v2-centos7-gcc11-opt/ ...

This is a problem because it hides many valid warnings. We might use -isystem for Gaudi but even that is probably not a good idea.

@clemenci do you know why this happens and how to fix it?