DuneMPI: append flags to `COMPILE_OPTIONS` instead of `COMPILE_FLAGS`

COMPILE_FLAGS is a string, but set_property(... APPEND ...) appends to a list. If the list of compile flags is not empty this results in a superfluous semicolon in the compile flags which will break the compiler invocation.

For example, when MPI_DUNE_COMPILE_FLAGS=-lpthread, calling add_dune_mpi_flags twice for the same target would result in the compiler command containing -lpthread;-lpthread.

This patch makes use of the COMPILE_OPTIONS property instead which is a list.