fltk: replace dependency on X.org server with dependency on libraries

fltk is a X client library, so it doesn't make sense for it to depend
on the X.org server. Instead, it depends on libX11, libXext and libXt,
so we use those libraries as fltk dependencies.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>