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/condition_variable b/include/condition_variable
index dc67266..1af2484 100644
--- a/include/condition_variable
+++ b/include/condition_variable
@@ -115,6 +115,8 @@
 #pragma GCC system_header
 #endif
 
+#ifndef _LIBCPP_HAS_NO_THREADS
+
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 class _LIBCPP_TYPE_VIS condition_variable_any
@@ -253,4 +255,6 @@
 
 _LIBCPP_END_NAMESPACE_STD
 
+#endif // !_LIBCPP_HAS_NO_THREADS
+
 #endif  // _LIBCPP_CONDITION_VARIABLE