message: GCC doesn't like this flag.
diff: @@ -14,9 +14,9 @@ $(patsubst %.stan,%,$(wildcard $(addsuffix .stan,$(MAKECMDGOALS)))) : %$(EXE) :
  @echo ''
  @echo '--- Linking C++ model ---'
  ifneq (,$(findstring allow_undefined,$(STANCFLAGS)))
  - $(LINK.cc) -include-pch $(MODEL_PCH) -O$O $(OUTPUT_OPTION) $(CMDSTAN_MAIN) -include $< -include $(USER_HEADER) $(LIBCVODES)
  + $(LINK.cc) $(if $(filter $(CC_TYPE),clang++),-include-pch $(MODEL_PCH)) -O$O $(OUTPUT_OPTION) $(CMDSTAN_MAIN) -include $< -include $(USER_HEADER) $(LIBCVODES))
  else
  - $(LINK.cc) -include-pch $(MODEL_PCH) -O$O $(OUTPUT_OPTION) $(CMDSTAN_MAIN) -include $< $(LIBCVODES)
  + $(LINK.cc) $(if $(filter $(CC_TYPE),clang++),-include-pch $(MODEL_PCH)) -O$O $(OUTPUT_OPTION) $(CMDSTAN_MAIN) -include $< $(LIBCVODES)
  endif
  .PRECIOUS: %.hpp
  