change the way MPI datatypes are declared.

change the way MPI datatypes are declared.
Our previous solution did not let everything seen as const, and it was preventing some codes to compile (initializer element is not constant).
This allows to compile  https://github.com/open-mpi/mpi-test-suite.git without changint the code (will be added in proxy apps) in tst_types.c
todo : other types (op and co)