message: add the flag if STAN_OPENCL is defined
diff: @@ -27,6 +27,10 @@ ifneq ($(findstring allow_undefined,$(STANCFLAGS)),)
  $(STAN_TARGETS) examples/bernoulli/bernoulli$(EXE) $(patsubst %.stan,%$(EXE),$(wildcard src/test/test-models/*.stan)) : CXXFLAGS_PROGRAM += -include $(USER_HEADER)
  endif
  +ifdef STAN_OPENCL
  +STANCFLAGS+= --use-opencl
  +endif
  +
  %.hpp : %.stan bin/stanc$(EXE)
  @echo ''
  @echo '--- Translating Stan model to C++ code ---'
  