message: update make/stanc
diff: ################################################################################
  # stanc build rules
  +# if nothing was set to $(OS) that means that the Stan Math submodule is missing
  +# used to creat breaks in error messages
  +define n
  +
  +
  +endef
  +
  +OS ?= missing-submodules
  +CMDSTAN_SUBMODULES = 1
  +
  ifeq ($(OS),Windows_NT)
  OS_TAG := windows
  else ifeq ($(OS),Darwin)
  OS_TAG := mac
  else ifeq ($(OS),Linux)
  OS_TAG := linux
  +else ifeq ($(OS),missing-submodules)
  + CMDSTAN_SUBMODULES = 0
  else
  - $(error Can't detect OS properly. This will impede automatically downloading the correct stanc. Please visit https://github.com/stan-dev/stanc3/releases and download a stanc binary for your OS and place it in ./bin/stanc)
  + $(error $n Can't detect OS properly. $n This will impede automatically downloading the correct stanc. $n Please visit https://github.com/stan-dev/stanc3/releases and download a stanc binary for your OS and place it in ./bin/stanc)
  endif
  -
  +ifeq ($(CMDSTAN_SUBMODULES),1)
  ifeq ($(STANC3),)
  bin/stanc$(EXE) :
  @mkdir -p $(dir $@)
  @@ -47,3 +59,4 @@ $(patsubst %.cpp,%.d,$(STANC_TEMPLATE_INSTANTIATION_CPP)) : DEPTARGETS = -MT $(p
  ifneq ($(filter bin/stanc$(EXE) bin/stan/libstanc.a $(STANC_TEMPLATE_INSTANTIATION),$(MAKECMDGOALS)),)
  -include $(patsubst %.cpp,%.d,$(STANC_TEMPLATE_INSTANTIATION_CPP))
  endif
  +endif
  \ No newline at end of file
  