MarlinFastJet package cannot be built in Key4hep environment

[tmadlener]

Just ran into this with @madbaron. For some reason MarlinFastJet cannot be built when sourcing the generated setup script but seems to build fine when done via spack. The reason is that apparently the includes for the fastjet contribs are not properly propagated.

To reproduce:

source /cvmfs/sw-nightlies.hsf.org/key4hep/setup.sh
git clone https://github.com/iLCSoft/MarlinFastJet
cd MarlinFastJet
cmake -B build -S . -DCMAKE_CXX_STANDARD=20
cmake --build build

which will lead to

[ 25%] Building CXX object CMakeFiles/MarlinFastJet.dir/src/FastJetClustering.cc.o
[ 50%] Building CXX object CMakeFiles/MarlinFastJet.dir/src/FastJetProcessor.cpp.o
In file included from /afs/desy.de/user/m/madlener/private/work/tmp/MarlinFastJet/src/FastJetProcessor.cpp:10:
/afs/desy.de/user/m/madlener/private/work/tmp/MarlinFastJet/./include/FastJetUtil.h:48:10: fatal error: fastjet/contrib/ValenciaPlugin.hh: No such file or directory
   48 | #include <fastjet/contrib/ValenciaPlugin.hh>
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
compilation terminated.

It's currently not clear to me if this is a spack problem, a fastjet problem, a pkg-config problem in cmake, or simply just something that needs to be solved in MarlinFastJet.