PkgConfig and ArborX installation

Created by: aprokop

If

pkg_check_modules(PC_Kokkos QUIET kokkos) 

fails in FindKokkos.cmake, the proper PC_Kokkos_CFLAGS are not set, and the whole Kokkos target becomes half-baked target. However, for downstream projects that compile against ArborX this fact may be obscured, and lead to errors like

#error "You enabled Kokkos OpenMP support without enabling OpenMP in the compiler!"

My question is: why do we not automatically fail when pkg_check_modules fails? Are there situations where we would want to continue?