Fix variable change check in CMake

[tim-griesbach]

Fix variable change check in CMake

Problem description

Reconfiguring with enabled MPI after starting from a CMake configuration with MPI disabled results in a setup with enabled MPI but deactivated MPI I/O, which is deprecated. The expected behavior is that MPI I/O is enabled if MPI is enabled.

Proposed changes

The issue arises because $CACHE{SC_ENABLE_MPI} reflects the current MPI setting rather than the previous one. As a result, the condition

NOT "${SC_ENABLE_MPI}" STREQUAL "$CACHE{SC_ENABLE_MPI}"

evaluates to false, preventing the check for MPI I/O when MPI was previously disabled. To address this, I introduced a new variable, CACHED_SC_ENABLE_MPI, to track the last MPI setting, ensuring that the last used value of SC_ENABLE_MPI is preserved.