[libcxx] Attempt to fix the gcc build failure

[rmaprath]

Summary

This looks like to be caused by a bug in the gcc pre-processor in some older versions of gcc (saw it on 4.8.3) [1].

Attempt to fix it by making the patch a bit more verbose.

[1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63662

[aizatsky]

Thanks.

include/__threading_support

 #define _LIBCPP_THREADING_SUPPORT
 
 #include <__config>
 
 #ifndef _LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER
 #pragma GCC system_header
 #endif
 
 #ifndef _LIBCPP_HAS_NO_THREADS
 
-#if defined(_LIBCPP_HAS_THREAD_API_EXTERNAL) && (!defined(__has_include) || __has_include(<__external_threading>))
-#include <__external_threading>
+// some versions of gcc does not like it when the following two #if conditions
+// are combined into a single #if line, hence the redundancy.
+#if defined(_LIBCPP_HAS_THREAD_API_EXTERNAL)
+#  if !defined(__has_include) || __has_include(<__external_threading>)
+#    include <__external_threading>
+#  else
+#    include <pthread.h>
+#    include <sched.h>
+#  endif
 #else
-#include <pthread.h>
-#include <sched.h>
+#  include <pthread.h>
+#  include <sched.h>
 #endif
 
 #if defined(_LIBCPP_HAS_THREAD_API_EXTERNAL)
 #define _LIBCPP_THREAD_ABI_VISIBILITY _LIBCPP_FUNC_VIS
 #else
 #define _LIBCPP_THREAD_ABI_VISIBILITY inline _LIBCPP_INLINE_VISIBILITY
 #endif
 
 _LIBCPP_BEGIN_NAMESPACE_STD