(as of 2.7.1) configure has this option:
--with-cairo use cairo (and pango) if available [yes]
So currently it's an automagical dependency with is a Bad Thing (TM) :) and
should be converted into a USE flag.
And why is it useful? You can plot to for example png files on headless
machine, which otherwise requires an X connection. Took me a while to find out
why it somewhere works (Fedora, my Gentoo laptop) and somewhere not... and it's
this automagic dependency.
As help("png") says:
R can be compiled without support for each of these devices: this
will be reported if you attempt to use them on a system where they
are not supported. For 'type = "Xlib"' they may not be usable
unless the X11 display is available to the owner of the R process.
'type = "cairo"' requires cairo 1.2 or later. 'type = "quartz"'
uses the 'quartz' device and so is only available where that is
(on some Mac OS X) builds. It is at present experimental.
And when testing this, I also noticed that cairo/pango needs to be built with X
use flag, otherwise R configure script says "checking whether cairo including
pango is >= 1.0 and works... no". So there would be a need for build_with_use,
sadly. This adds some extra X library deps but still you don't need an X
connection. I tested with latest stable versions of cairo/pango on amd64. I
don't know what's the difference between having only cairo and both
cairo/pango, probably doesn't need extra pango flag and cairo flag could depend
on both.
