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

llvm-svn: 217271
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: b3fcc67f8f13cd95d36ed29d0ae1308decb9e099
diff --git a/include/shared_mutex b/include/shared_mutex
index 00f816d..9b7f0bf 100644
--- a/include/shared_mutex
+++ b/include/shared_mutex
@@ -112,6 +112,10 @@
 #pragma GCC system_header
 #endif
 
+#ifdef _LIBCPP_HAS_NO_THREADS
+#error <shared_mutex> is not supported on this single threaded system
+#else // !_LIBCPP_HAS_NO_THREADS
+
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 class _LIBCPP_TYPE_VIS shared_timed_mutex
@@ -414,6 +418,8 @@
 
 _LIBCPP_END_NAMESPACE_STD
 
+#endif  // !_LIBCPP_HAS_NO_THREADS
+
 #endif  // _LIBCPP_STD_VER > 11
 
 #endif  // _LIBCPP_SHARED_MUTEX