mpiwrap: revamp the way the wrappers are generated
mpiwrap: revamp the way the wrappers are generated

There's just no pretty way of doing it (that I can come up with),
but this way makes it clearer how we switch based on ADIOS2_HAVE_MPI,
and it doesn't required the namespaced mpidummy in the
!ADIOS2_HAVE_MPI case.

ornladios/ADIOS2,JasonRuonanWang/ADIOS2,JasonRuonanWang/ADIOS2,JasonRuonanWang/ADIOS2,ornladios/ADIOS2,ornladios/ADIOS2,JasonRuonanWang/ADIOS2,JasonRuonanWang/ADIOS2,ornladios/ADIOS2,ornladios/ADIOS2