Add a declaration of environ global for Darwin

llvm-svn: 16100
diff --git a/llvm/lib/System/Unix/Program.cpp b/llvm/lib/System/Unix/Program.cpp
index 6943d52..ca8514d 100644
--- a/llvm/lib/System/Unix/Program.cpp
+++ b/llvm/lib/System/Unix/Program.cpp
@@ -24,6 +24,8 @@
 #include <sys/wait.h>
 #endif
 
+extern char** environ;
+
 namespace llvm {
 using namespace sys;