diff -wpruN '--exclude=*.orig' a~/lib/Basic/Targets/OSTargets.h a/lib/Basic/Targets/OSTargets.h
--- a~/tools/clang/lib/Basic/Targets/OSTargets.h	1970-01-01 00:00:00
+++ a/tools/clang/lib/Basic/Targets/OSTargets.h	1970-01-01 00:00:00
@@ -624,6 +624,7 @@ protected:
   void getOSDefines(const LangOptions &Opts, const llvm::Triple &Triple,
                     MacroBuilder &Builder) const override {
     DefineStd(Builder, "sun", Opts);
+    Builder.defineMacro("__illumos__");
     DefineStd(Builder, "unix", Opts);
     Builder.defineMacro("__ELF__");
     Builder.defineMacro("__svr4__");