gtk+: Add PACKAGECONFIG for directfb

* building without x11 doesn't work, because it sets default
  gdkbackend to x11 and then requires cairo-xlib to be available
* checking for CAIRO_BACKEND... no
  configure: error: Package requirements (cairo-xlib >= 1.6) were not met:

  No package 'cairo-xlib' found

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>