GCC 4.8's parser ICEs on this template

Simply ifdef this function out for GCC < 4.9. Then
is_functor_argument_immutable will always answer false, but that's the
conservative answer, so it should be fine.

Signed-off-by: Matthias Kretz <kretz@kde.org>
unit test for is_functor_argument_immutable

Signed-off-by: Matthias Kretz <kretz@kde.org>
traits: fix is_functor_argument_immutable implementation

my initial test code was wrong and I didn't notice that the trait wasn't
actually working. The idea was correct - just not correctly implemented.
Now it works. Even with GCC 4.8.

Signed-off-by: Matthias Kretz <kretz@kde.org>