message: Add a platform check in Makefile (all)
diff: @@ -103,7 +103,13 @@ libs=-framework Carbon \
  cxx=g++ -fprofile-arcs -ftest-coverage -std=gnu++0x
  +UNAME_S := $(shell uname -s)
  +ifeq ($(UNAME_S),Darwin)
  default: fmt app
  +else
  +default:
  + $(error this platform is not supported by Makefile.)
  +endif
  csapi: generate_cs_api fmt
  