Draft: use PATH_SUFFIXES instead of HINT for finding system Eigen

Hi everyone,

On my system (linux + CMake v3.22) the ENABLE_SYSTEM_CONTRIB=1 option does not lead to the desired effect (i.e. it uses contrib/Eigen instead of system Eigen).

If I understand the manual of CMake [1] correctly, subdirectories are specified with PATH_SUFFIXES and not HINTS (in this case HINT would be /usr/include, if I got it right).

It works with this patch, at least on my system and for Eigen ;-). I guess the same should apply to robin_hood... That said, I haven't installed it system wide, so I cannot check.

Cheers, Nicolas.

[1] https://cmake.org/cmake/help/latest/command/find_path.html