message: ensure all MPI things get defined during test model builds
diff: @@ -113,7 +113,7 @@ stan/%.o : stan/%.cpp
  @echo ''
  @echo '--- Linking C++ model ---'
  @test -f $(dir $<)USER_HEADER.hpp || touch $(dir $<)USER_HEADER.hpp
  - $(LINK.cc) $(CMDSTAN_MAIN) -O$O $(OUTPUT_OPTION) -include $< -include $(dir $<)USER_HEADER.hpp $(LIBSUNDIALS) $(LDFLAGS_MPI)
  + $(LINK.cc) $(CMDSTAN_MAIN) -O$O $(OUTPUT_OPTION) -include $< -include $(dir $<)USER_HEADER.hpp $(LIBSUNDIALS) $(CXXFLAGS_MPI) $(LDFLAGS_MPI)
  ##
  # Tell make the default way to compile a .o file.
  