libcxx: Define __STDCPP_THREADS__ to 1, not to __cplusplus.

[cpp.predefined]p2:

   __STDCPP_THREADS__
    Defined, and has the value integer literal 1, if and only if a program
    can have more than one thread of execution .

Also define it only if it's not defined already, since it's supposed
to be defined by the compiler.

Also move it from thread to __config (which requires setting it only
if _LIBCPP_HAS_NO_THREADS is not defined).

Part of PR33230. The intent is to eventually make the compiler define
this instead.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@367316 91177308-0d34-0410-b5e6-96231b3b80d8

include/__config

@@ -1097,6 +1097,14 @@ _LIBCPP_FUNC_VIS extern "C" void __sanitizer_annotate_contiguous_container(
        _LIBCPP_HAS_NO_THREADS is defined.
 #endif
 
+#if defined(__STDCPP_THREADS__) && defined(_LIBCPP_HAS_NO_THREADS)
+#error _LIBCPP_HAS_NO_THREADS cannot be set when __STDCPP_THREADS__ is set.
+#endif
+
+#if !defined(_LIBCPP_HAS_NO_THREADS) && !defined(__STDCPP_THREADS__)
+#define __STDCPP_THREADS__ 1
+#endif
+
 // The glibc and Bionic implementation of pthreads implements
 // pthread_mutex_destroy as nop for regular mutexes. Additionally, Win32
 // mutexes have no destroy mechanism.

include/thread

@@ -14,8 +14,6 @@
 
     thread synopsis
 
-#define __STDCPP_THREADS__ __cplusplus
-
 namespace std
 {
 
@@ -107,8 +105,6 @@ void sleep_for(const chrono::duration<Rep, Period>& rel_time);
 _LIBCPP_PUSH_MACROS
 #include <__undef_macros>
 
-#define __STDCPP_THREADS__ __cplusplus
-
 #ifdef _LIBCPP_HAS_NO_THREADS
 #error <thread> is not supported on this single threaded system
 #else // !_LIBCPP_HAS_NO_THREADS

test/std/thread/macro.pass.cpp

@@ -10,7 +10,7 @@
 
 // <thread>
 
-// #define __STDCPP_THREADS__ __cplusplus
+// #define __STDCPP_THREADS__ 1
 
 #include <thread>
 
@@ -20,6 +20,8 @@ int main(int, char**)
 {
 #ifndef __STDCPP_THREADS__
 #error __STDCPP_THREADS__ is not defined
+#elif __STDCPP_THREADS__ != 1
+#error __STDCPP_THREADS__ has the wrong value
 #endif
 
   return 0;