C++ compiler in configure. If we have a C++ compiler, then the MPI
C++ bindings are built by default. If we don't have a C++ compiler,
then the MPI C++ bindings are not built by default.
--enable-mpi-cxx will now force an error if there is no C++ compiler
available. --disable-mpi-cxx (or the lack of a C++ compiler) will now
disable many of the C++ compiler checks in configure.
Note that there are a few items to clean up regarding the difference
between C's _Bool type and C++'s bool type. Right now, we assume that
they are the same. But they aren't, and they shouldn't be treated as
such. This cleanup will be forced in MPI-2.2 with the introduction of
the MPI_C_BOOL MPI datatype.
This commit was SVN r21755.
