message: define missing OS variable early enough in makefiles
diff: @@ -25,6 +25,11 @@ RAPIDJSON ?= lib/rapidjson_1.1.0/
  INC_FIRST ?= -I src -I $(STAN)src -I $(RAPIDJSON)
  USER_HEADER ?= $(dir $<)user_header.hpp
  +## Detect operating system
  +ifneq ($(OS),Windows_NT)
  + OS := $(shell uname -s)
  +endif
  +
  ## Set default compiler
  ifeq (default,$(origin CXX))
  ifeq ($(OS),Darwin) ## Darwin is Mac OS X
  