scanner: make use of __has_attribute()

A more generic way to evaluating various attributes, __has_attribute is
available with gcc, clang, even the Oracle/Sun compiler.

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Derek Foreman <derekf@osg.samsung.com>