[cairo] Conditional compilation of backends

[Carl Worth]

I just committed to CVS a bit of build work to standardize the
configure options for selecting which backends will be compiled. The
two options that are now available are:

	--disable-xlib
	--disable-ps

The --disable-xlib option replaces the previous --without-x option.

It's now a very simple job to add new optional backends to cairo. It
basically requires one new .c file, a couple of guarded prototypes in
cairo.h, and a cut-and-paste job in configure.in, (which could even be
simplified in the hands of an m4 wizard).

I hope to add the new png and glx (or whatever it's called) backends
soon.

-Carl