qt5: fix to work on LLVM 4

(this is reapplying 6b1957d1, which got
lost in a merge)

Including apple_sdk.sdk is generally a recipe for a bad time on LLVM 3.8
and above, since you end up with bad headers in the wrong place that hurt
the new libc++ in 3.8 and above. In this case, qt only wanted the super-
generic SDK for CUPS headers, which we can just depend on directly now.