http://bugs.winehq.org/show_bug.cgi?id=34376
--- Comment #1 from Nikolay Sivov bunglehead@gmail.com 2013-08-28 03:29:07 CDT --- Hi, Andrew.
I heard about that on #winehackers already, it turns out to be Gentoo issue - provided headers do not match library. You could easily check if your libxml2 headers have that symbol. Wine only uses it when LIBXML2_NEW_BUFFER is defined.