tigervnc: fix ctrl-* keys, eg ctrl-c ctrl-v ctrl-d by applying patch to fltk

I'm unsure about
- is it ok to apply this patch globally, or should it be overridden for tigervnc only?
- how many rebuilds it triggers
- whether it was neccessary to update to latest dev version (seems to work)
The bug report indicates that others distros just includes this patch (?)

Cf. #1498. vcunat: the patch should be usable globally, as e.g. Arch does it.
Using autoreconfHook doesn't work for me, maybe --force is bad for it.