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In commit e374b9f1 (FindMPI: Restore MPI_<LANG>_COMPILE_FLAGS as a command-line string, 2018-09-10, v3.12.3~6^2) we tried to restore the compile flags but accidentally inverted the arguments to `list(JOIN)` causing both `MPI_<LANG>_COMPILE_FLAGS` and `MPI_<LANG>_COMPILE_OPTIONS` to be empty. Issue: #18349
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