[CMake] Use MPI_FOUND instead of MPI_CXX_FOUND

For binary compatibility, Dune only uses the C bindings
of MPI. MPI_FOUND is deprecated from CMake but Dune does
defines it and encourage its use.
(reviewed by timok)

test/porousmediumflow/richards/implicit/CMakeLists.txt

 add_input_file_links()
 
-if(MPI_CXX_FOUND)
+if(MPI_FOUND)
   add_dumux_test(test_boxrichards test_boxrichards test_boxrichards.cc
                  python ${CMAKE_SOURCE_DIR}/bin/runtest.py
                    --script fuzzy