FindMPI: Regression when re-running on existing build tree
The change in !4153 (merged) only works on the first run in a build tree. After that, the cache entries we save have different parts of the original list of include directories split up as before. On runs after the first one, we now have two problems:
- The original list is not re-assembled into
- The cache entries we save for
MPI_<LANG>_ADDITIONAL_INCLUDE_DIRS, etc. no longer preserve the order so we do not have enough information to re-assemble them.