diff --git a/llvm/lib/Support/Unix/Program.inc b/llvm/lib/Support/Unix/Program.inc
--- a/llvm/lib/Support/Unix/Program.inc
+++ b/llvm/lib/Support/Unix/Program.inc
@@ -329,6 +329,11 @@
   return true;
 }
 
+#ifdef _AIX
+extern "C" pid_t (wait4)(pid_t pid, int *status, int options,
+                         struct rusage *usage);
+#endif
+
 namespace llvm {
 
 ProcessInfo sys::Wait(const ProcessInfo &PI, unsigned SecondsToWait,