qt: mark as obsolete

qt 4.x is no longer supported upstream since December 2015:

https://blog.qt.io/blog/2015/05/26/qt-4-8-7-released/



So it shouldn't be used for new projects.  Add an obsolete warning to the
package prompt to make this clear.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>