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/mutex.cpp

@@ -14,6 +14,7 @@
 #include "cassert"
 
 _LIBCPP_BEGIN_NAMESPACE_STD
+#ifndef _LIBCPP_HAS_NO_THREADS
 
 const defer_lock_t  defer_lock = {};
 const try_to_lock_t try_to_lock = {};
@@ -206,18 +207,42 @@ recursive_timed_mutex::unlock() _NOEXCEPT
     }
 }
 
+#endif // !_LIBCPP_HAS_NO_THREADS
+
 // If dispatch_once_f ever handles C++ exceptions, and if one can get to it
 // without illegal macros (unexpected macros not beginning with _UpperCase or
 // __lowercase), and if it stops spinning waiting threads, then call_once should
 // call into dispatch_once_f instead of here. Relevant radar this code needs to
 // keep in sync with:  7741191.
 
+#ifndef _LIBCPP_HAS_NO_THREADS
 static pthread_mutex_t mut = PTHREAD_MUTEX_INITIALIZER;
 static pthread_cond_t  cv  = PTHREAD_COND_INITIALIZER;
+#endif
 
 void
 __call_once(volatile unsigned long& flag, void* arg, void(*func)(void*))
 {
+#if defined(_LIBCPP_HAS_NO_THREADS)
+    if (flag == 0)
+    {
+#ifndef _LIBCPP_NO_EXCEPTIONS
+        try
+        {
+#endif  // _LIBCPP_NO_EXCEPTIONS
+            flag = 1;
+            func(arg);
+            flag = ~0ul;
+#ifndef _LIBCPP_NO_EXCEPTIONS
+        }
+        catch (...)
+        {
+            flag = 0ul;
+            throw;
+        }
+#endif  // _LIBCPP_NO_EXCEPTIONS
+    }
+#else // !_LIBCPP_HAS_NO_THREADS
     pthread_mutex_lock(&mut);
     while (flag == 1)
         pthread_cond_wait(&cv, &mut);
@@ -248,6 +273,8 @@ __call_once(volatile unsigned long& flag, void* arg, void(*func)(void*))
     }
     else
         pthread_mutex_unlock(&mut);
+#endif // !_LIBCPP_HAS_NO_THREADS
+
 }
 
 _LIBCPP_END_NAMESPACE_STD