Disable Quartz backend on Darwin

Quartz backend interacts badly with our library stack. The most notable
issue is that when quartz-font is enabled, cairo will never use fontconfig
but instead uses CoreGraphics API to find fonts in system-default font
paths; as a result, any fonts installed with pkgsrc will never be found.
OTOH fontconfig by default searches for fonts in MacOS X system-default
paths too so sticking with it will not cause a problem.

(pho)