Add _ATTRIBUTE macro for gcc: Justin Hibbits

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@132334 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/include/__config b/include/__config
index 86ce000..fea7a96 100644
--- a/include/__config
+++ b/include/__config
@@ -183,6 +183,8 @@
 
 #elif defined(__GNUC__)
 
+#define _ATTRIBUTE(x) __attribute__((x))
+
 #if !__EXCEPTIONS
 #define _LIBCPP_NO_EXCEPTIONS
 #endif