XQuartz: detect Tiger's prefix

Library/Homebrew/os/mac/xquartz.rb

@@ -99,6 +99,9 @@ module OS
           Pathname.new("/opt/X11")
         elsif Pathname.new("/usr/X11/lib/libpng.dylib").exist?
           Pathname.new("/usr/X11")
+        # X11 doesn't include libpng on Tiger
+        elsif Pathname.new("/usr/X11R6/lib/libX11.dylib").exist?
+          Pathname.new("/usr/X11R6")
         end
       end