directfb: allow SVG support

directfb: allow SVG support

DirectFB SVG support rely on Cairo and Cairo depends on DirectFB.

Since Cairo DirectFB backend is marked as experimental, this patch choose to
disable this backend when DirectFB SVG support is enable.

In add, this patch may solve some tricky situations between DirectFB and Cairo
(see http://sysmic.org/~jezz/results/c2f11fc0) and improve build reproducibility.

[Peter: propagate atomics dependency, needs libsvg-cairo, add cairo includes]
Signed-off-by: Jérôme Pouiller <jezz@sysmic.org>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>