universal variant and lib: dependencies

[Jeremy Huddleston]

When we use lib: dependencies, we search ${x11prefix}, but when we use  
+universal, we -isysroot /some/path.sdk

There are some libs that are can be present in ${x11prefix} but not in  
the SDK.  For instance, libpixman, libcairo, and xcb-util might be  
satisfied by ${x11prefix} from a user with XQuartz-2.3.2 installed,  
but those aren't in the actual 10.5.0 SDK that +universal pulls in.

Should the search path be altered if we have +universal?