USE=xml should be a general use enabling XML files support. Then USE=expat (or USE=libxml2) should work as a specific USE switching between the preferred and discouraged implementation.
You want it, you do it ;)
Created attachment 278933 [details, diff] Patch fixing the issue (In reply to comment #1) > You want it, you do it ;) Like this?
I more thought that you would commit it in... Anyway done, with slight difference preventing theoretical automagicness.
(In reply to comment #3) > I more thought that you would commit it in... Sorry, I completely forgot about it.