COMP: guards around #define OMPI_SKIP_MPICXX (avoids compiler warnings)

applications/test/00-dummy/library/mpi/dummyMpiLib.cpp

applications/test/parallel-comm2/Test-parallel-comm2.C

applications/test/parallel-comm3a/Test-parallel-comm3a.C

...test/parallel-external-init/Test-parallel-external-init.C

applications/test/processorTopology/Test-processorTopology.C

src/Pstream/mpi/PstreamGlobals.H

src/Pstream/mpi/UPstreamWrapping.H

src/parallel/decompose/kahipDecomp/kahipDecomp.C

src/parallel/decompose/metisDecomp/metisDecomp.C

src/parallel/decompose/ptscotchDecomp/ptscotchDecomp.C

src/parallel/decompose/scotchDecomp/scotchDecomp.C

src/renumber/zoltanRenumber/zoltanRenumber.C