__has_feature crutch

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@115540 91177308-0d34-0410-b5e6-96231b3b80d8

include/__config

diff --git a/include/__config b/include/__config
index 9540a11..7cc07fa 100644
--- a/include/__config
+++ b/include/__config
@@ -249,4 +249,8 @@
 #define constexpr const
 #endif
 
+#ifndef __has_feature(__x)
+#define __has_feature(__x) 0
+#endif
+
 #endif  // _LIBCPP_CONFIG