unittests/SupportTests/Initialize.MultipleThreads: Enable pthread_attr_setstack(3) only on Linux.

I got blamed on darwin11;
unittests/Support/ManagedStatic.cpp:35: error: 'pthread_attr_setstack' was not declared in this scope

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@173355 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/unittests/Support/ManagedStatic.cpp b/unittests/Support/ManagedStatic.cpp
index b1df2bd..a413761 100644
--- a/unittests/Support/ManagedStatic.cpp
+++ b/unittests/Support/ManagedStatic.cpp
@@ -32,7 +32,9 @@
   void *allocate_stack(pthread_attr_t &a, size_t n = 65536) {
     void *stack = malloc(n);
     pthread_attr_init(&a);
+#if defined(__linux__)
     pthread_attr_setstack(&a, stack, n);
+#endif
     return stack;
   }
 }