HAVE_CXX11_PERFECT_FORWARDING doesn't seem to work against libc++

(Just one small fix for building against libc++, an unknown amount of
more difficult issues left to solve.)

Change-Id: I9789b8d76aa214558ab4baad823b6650ebc640d3

include/rtl/allocator.hxx

@@ -127,7 +127,7 @@ public:
     }
 
     //-----------------------------------------
-#if HAVE_CXX11_PERFECT_FORWARDING
+#if HAVE_CXX11_PERFECT_FORWARDING && !defined(_LIBCPP_VERSION)
     template< typename... Args >
     void construct (pointer p, Args &&... value)
     {