cmake: use MPI_CXX for targets of CXX sources #5252

Job	Run time
git_checks	8s
format	25s
linux_el8 (el8, icc, mpich)	23m 24s
windows (win2019, shared, serial)	11m 18s
linux_el8 (el8, oneapi, mpich)	21m 40s
CodeQL (cpp)	21m 23s
windows (win2019, shared, ompi)	16m 49s
docker (ubuntu-bionic)	7m 25s
windows (win2022, shared, serial)	11m 50s
windows (win2022, shared, ompi)	15m 53s
linux_ubuntu (ubuntu20.04, gcc8, shared, mpich)	14m 34s
windows (win2022, static, serial, windows-2022, vs2022)	13m 0s
macos (macos-13, shared)	6m 2s
linux_ubuntu (ubuntu20.04, gcc9, shared, mpich)	17m 40s
macos (macos-14, static)	3m 55s
linux_ubuntu (ubuntu20.04, gcc10, shared, mpich)	17m 31s
macos (macos-14, shared)	3m 6s
linux_ubuntu (ubuntu20.04, gcc11, shared, mpich)	17m 43s
linux_ubuntu (ubuntu20.04, clang6, shared, mpich)	11m 18s
linux_ubuntu (ubuntu20.04, clang10, shared, mpich)	14m 46s
linux_ubuntu (ubuntu20.04, gcc8, ompi)	33m 56s
linux_ubuntu (ubuntu20.04, gcc8, serial)	9m 40s
linux_ubuntu (ubuntu20.04, clang6, ompi)	30m 42s
linux_ubuntu (ubuntu20.04, clang6, serial)	8m 6s
linux_ubuntu (ubuntu20.04, gcc8, static, ompi, build_only)	7m 55s
linux_ubuntu (ubuntu20.04, clang6, static, ompi, build_only)	4m 51s
linux_ubuntu (ubuntu20.04, gcc8, static, serial)	9m 21s
linux_ubuntu (ubuntu22.04, rocm, serial, build_only)	3m 11s
contract (examples)	2m 14s
contract (lammps)	4m 13s
contract (tau)	10m 52s
build_and_test	1s
	6h 14m 52s

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