[PATCH] libstdc++: Only use __gthread_yield if gthreads is available

[Matheus Castanho]

When building libstdc++ with --disable-threads g++ complains that no
__gthread_yield declaration is available. The code should check
_GLIBCXX_HAS_GTHREADS before trying to use it.

libstdc++-v3/ChangeLog:

        * include/std/mutex (__lock_impl): Check _GLIBCXX_HAS_GTHREADS before
        using __gthread_yield
---
 libstdc++-v3/include/std/mutex | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/libstdc++-v3/include/std/mutex b/libstdc++-v3/include/std/mutex
index eeb51fdb840..7ab4ee12690 100644
--- a/libstdc++-v3/include/std/mutex
+++ b/libstdc++-v3/include/std/mutex
@@ -618,7 +618,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 		      return;
 		    }
 		}
-#ifdef _GLIBCXX_USE_SCHED_YIELD
+#if defined _GLIBCXX_HAS_GTHREADS && defined _GLIBCXX_USE_SCHED_YIELD
 		__gthread_yield();
 #endif
 		constexpr auto __n = 1 + sizeof...(_L1);
-- 
2.31.1