OpenMPI: Add option to support building using C++11 compiler.

* Only enabled for OpenMPI 1.7+ since 1.6 branch fails tests when
  building with Clang.
* Add patch to fix C++11 reserved string literals.
  See also https://svn.open-mpi.org/trac/ompi/ticket/3704.
* Error and abort when building with C++11 using non-Clang compiler.