[OE-core] X11 as DISTRO_FEATURE

[Phil Blundell]

Right now, there are a bunch of recipes (e.g. dbus, cairo) which always
build against X11 even though the underlying software can work without
it.  This makes it somewhat painful to build a display system using
clutter (for example) without X.

It's also moderately fiddly to stop X11 from leaking in by mistake; you
can't just BBMASK the whole of "xorg-*" because things like pixman are
still needed by cairo even when not using X11.

So, to combat this, I would like to make X11 be a DISTRO_FEATURE and
arrange for:

a) libx11 to skip itself; and

b) recipes for which x11 is optional to not depend on X stuff 

if that feature isn't selected.

Any objections, better suggestions, ...?

p.