cmake mpi fixes

CMakeLists.txt

@@ -727,12 +727,8 @@ if(ENABLE_MPI)
   find_package(MPI)
   if(MPI_FOUND)
     set_config_option(HAVE_MPI "MPI")
-    list(APPEND EXTERNAL_INCLUDES ${MPI_INCLUDE_DIR})
-    list(APPEND EXTERNAL_LIBRARIES ${MPI_LIBRARIES})
-    include(CMakeForceCompiler)
-    # Warning: this actually requires cmake >= 2.8.5
-    cmake_force_c_compiler(${MPI_C_COMPILER} "MPI C Compiler")
-    cmake_force_cxx_compiler(${MPI_CXX_COMPILER} "MPI C++ Compiler")
+    list(APPEND EXTERNAL_INCLUDES ${MPI_CXX_INCLUDE_PATH})
+    list(APPEND EXTERNAL_LIBRARIES ${MPI_CXX_LIBRARIES})
   endif(MPI_FOUND)
 endif(ENABLE_MPI)