Only use <atomic> for C++11 and newer.

Any pre-C++11 clients of stdatomic.h that use libc++ are being forced
over to <atomic>, which they don't have the language support to use.

Change-Id: I62445c1f2541410a1569498c09433c7196635537
diff --git a/libc/include/stdatomic.h b/libc/include/stdatomic.h
index bcea859..58cb1bc 100644
--- a/libc/include/stdatomic.h
+++ b/libc/include/stdatomic.h
@@ -33,7 +33,7 @@
 #include <sys/cdefs.h>
 
 
-#if defined(__cplusplus) && defined(_USING_LIBCXX)
+#if defined(__cplusplus) && __cplusplus >= 201103L && defined(_USING_LIBCXX)
 # ifdef __clang__
 #  if __has_feature(cxx_atomic)
 #   define _STDATOMIC_HAVE_ATOMIC