Allow libc++ to be built on systems without POSIX threads

If you're crazy enough to want this sort of thing, then add
-D_LIBCPP_HAS_NO_THREADS to your CXXFLAGS and
--param=additiona_features=libcpp-has-no-threads to your lit commnad line.

http://reviews.llvm.org/D3969


git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@217271 91177308-0d34-0410-b5e6-96231b3b80d8

src/ios.cpp

@@ -148,7 +148,7 @@ ios_base::getloc() const
 }
 
 // xalloc
-#if __has_feature(cxx_atomic)
+#if __has_feature(cxx_atomic) && !_LIBCPP_HAS_NO_THREADS
 atomic<int> ios_base::__xindex_ = ATOMIC_VAR_INIT(0);
 #else
 int ios_base::__xindex_ = 0;