For OpenMPI v 2+, add a configure option to provide C++ bindings. (#1730)

+ Starting with version 2.0, OpenMPI no longer provides C++ bindings by default
  (libmpi_cxx.so). Add a configure option to instruct the build to also build
  and install libmpi_cxx.so.
+ This MPI feature is needed by at least one spack package (moab).

var/spack/repos/builtin/packages/openmpi/package.py

@@ -165,6 +165,10 @@ def install(self, spec, prefix):
                        "--enable-shared",
                        "--enable-static"]
 
+        # for Open-MPI 2.0:, C++ bindings are disabled by default.
+        if self.spec.satisfies('@2.0:'):
+            config_args.extend(['--enable-mpi-cxx'])
+
         if getattr(self, 'config_extra', None) is not None:
             config_args.extend(self.config_extra)