Ensure typet constructor is always compatible with GCC 5's STL

When enabling NAMED_SUB_IS_FORWARD_LIST, compilation failed as the
std::forward_list constructors shipped with GCC 5 do not support an
empty initializer list. GCC 6 and later do not have any such problem.

Author: tautschnig

src/util/type.h

@@ -32,10 +32,18 @@ class typet:public irept
 
   explicit typet(const irep_idt &_id):irept(_id) { }
 
+#if !defined(__GNUC__) || __GNUC__ >= 6
   typet(irep_idt _id, typet _subtype)
     : irept(std::move(_id), {}, {std::move(_subtype)})
   {
   }
+#else
+  typet(irep_idt _id, typet _subtype)
+    : irept(std::move(_id))
+  {
+    subtype() = std::move(_subtype);
+  }
+#endif
 
   const typet &subtype() const
   {