Fix unavailability of __thread_id on external builds

[mclow.lists]

Summary

In fixing https://bugs.llvm.org/show_bug.cgi?id=42918 (r368916), I broke external threading support.
This should fix that

I don't have an external threading setup, so I'd appreciate it if someone who had one could test this.
I have tested with pthreads and with no-threads.

[ldionne]

The code _looks_ alright, but can @DavidSpickett please validate?

[mclow.lists]

This is an alternative to D66301

[DavidSpickett]

Fixes the build for us and is neater than my solution. LGTM

[mclow.lists]

Committed as revision 369399.

[DavidSpickett]

Apologies for the back and forth, I had my local builds mis-configured so the success was my mistake. I've put up a small change at https://reviews.llvm.org/D66518 that should fix things.

libcxx/include/__threading_support

 #include <errno.h>
 
 #ifndef _LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER
 #pragma GCC system_header
 #endif
 
 #if defined(_LIBCPP_HAS_THREAD_API_EXTERNAL)
 # include <__external_threading>
 #elif !defined(_LIBCPP_HAS_NO_THREADS)
 
-typedef ::timespec __libcpp_timespec_t;
-
 #if defined(_LIBCPP_HAS_THREAD_API_PTHREAD)
 # include <pthread.h>
 # include <sched.h>
 #endif
 
-_LIBCPP_PUSH_MACROS
-#include <__undef_macros>
-
 #if defined(_LIBCPP_HAS_THREAD_LIBRARY_EXTERNAL) || \
     defined(_LIBCPP_BUILDING_THREAD_LIBRARY_EXTERNAL) || \
     defined(_LIBCPP_HAS_THREAD_API_WIN32)
 #define _LIBCPP_THREAD_ABI_VISIBILITY _LIBCPP_FUNC_VIS
 #else
 #define _LIBCPP_THREAD_ABI_VISIBILITY inline _LIBCPP_INLINE_VISIBILITY
 #endif
 
 #if defined(__FreeBSD__) && defined(__clang__) && __has_attribute(no_thread_safety_analysis)
 #define _LIBCPP_NO_THREAD_SAFETY_ANALYSIS __attribute__((no_thread_safety_analysis))
 #else
 #define _LIBCPP_NO_THREAD_SAFETY_ANALYSIS
 #endif
 
+typedef ::timespec __libcpp_timespec_t;
+#endif // !defined(_LIBCPP_HAS_NO_THREADS)
+
+_LIBCPP_PUSH_MACROS
+#include <__undef_macros>
+
 _LIBCPP_BEGIN_NAMESPACE_STD
 
+#if !defined(_LIBCPP_HAS_NO_THREADS)
+
 #if defined(_LIBCPP_HAS_THREAD_API_PTHREAD)
 // Mutex
 typedef pthread_mutex_t __libcpp_mutex_t;
 #define _LIBCPP_MUTEX_INITIALIZER PTHREAD_MUTEX_INITIALIZER
 
 typedef pthread_mutex_t __libcpp_recursive_mutex_t;
 
 // Condition Variable
 typedef pthread_cond_t __libcpp_condvar_t;
 #define _LIBCPP_CONDVAR_INITIALIZER PTHREAD_COND_INITIALIZER

 
 // Thread
 #define _LIBCPP_NULL_THREAD 0U
 
 typedef void* __libcpp_thread_t;
 
 // Thread Local Storage
 typedef long __libcpp_tls_key;
 
 #define _LIBCPP_TLS_DESTRUCTOR_CC __stdcall
-#endif
+#endif // defined(_LIBCPP_HAS_THREAD_API_PTHREAD)
 
 // Mutex
 _LIBCPP_THREAD_ABI_VISIBILITY
 int __libcpp_recursive_mutex_init(__libcpp_recursive_mutex_t *__m);
 
 _LIBCPP_THREAD_ABI_VISIBILITY _LIBCPP_NO_THREAD_SAFETY_ANALYSIS
 int __libcpp_recursive_mutex_lock(__libcpp_recursive_mutex_t *__m);
 
 _LIBCPP_THREAD_ABI_VISIBILITY _LIBCPP_NO_THREAD_SAFETY_ANALYSIS
 bool __libcpp_recursive_mutex_trylock(__libcpp_recursive_mutex_t *__m);

 
 inline _LIBCPP_INLINE_VISIBILITY
 __thread_id
 get_id() _NOEXCEPT
 {
     return __libcpp_thread_get_current_id();
 }
 
 }  // this_thread
 
+#endif // !_LIBCPP_HAS_NO_THREADS
+
 _LIBCPP_END_NAMESPACE_STD
 
 _LIBCPP_POP_MACROS
 
-#endif // !_LIBCPP_HAS_NO_THREADS
-
 #endif // _LIBCPP_THREADING_SUPPORT