cairo: build with xcb support

XCB is a more "modern" way of communicating with X11 servers than
straight XLib.  It is required by some X11 applications.  Enabling
support in cairo should not affect applications which don't use it
explicitly.

Closes Homebrew/homebrew#9598.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>

Formula/cairo.rb

@@ -16,7 +16,8 @@ class Cairo < Formula
   def install
     system "./configure", "--disable-dependency-tracking",
                           "--prefix=#{prefix}",
-                          "--with-x"
+                          "--with-x",
+                          "--enable-xcb"
     system "make install"
   end
 end