force both C and CXX compiler when ENABLE_MPI is set

CMakeLists.txt

@@ -627,7 +627,8 @@ if(ENABLE_MPI)
      list(APPEND EXTERNAL_INCLUDES ${MPI_INCLUDE_DIR})
      list(APPEND EXTERNAL_LIBRARIES ${MPI_LIBRARIES})
      include(CMakeForceCompiler)
-     cmake_force_cxx_compiler(${MPI_COMPILER} "MPI C++ Compiler")
+     cmake_force_c_compiler(${MPI_C_COMPILER} "MPI C Compiler")
+     cmake_force_cxx_compiler(${MPI_CXX_COMPILER} "MPI C++ Compiler")
    endif(MPI_FOUND)
 endif(ENABLE_MPI)