Make sure we set configuration in back-deployed concurrency libraries.

Author: DougGregor

stdlib/public/Concurrency/Actor.cpp

@@ -36,6 +36,11 @@
 #include "swift/ABI/Actor.h"
 #ifndef SWIFT_CONCURRENCY_BACK_DEPLOYMENT
 #include "llvm/Config/config.h"
+#else
+// All platforms where we care about back deployment have a known
+// configurations.
+#define HAVE_PTHREAD_H 1
+#define SWIFT_OBJC_INTEROP 1
 #endif
 #include "llvm/ADT/PointerIntPair.h"
 #include "TaskPrivate.h"