Make sure libxml2 is built with iconv support.

This should fix the problem that libxml2 is built without libiconv and
gets dysfunctional on platforms where iconv(3) is not in libc and
libiconv is installed outside the system default directories.

I have confirmed that this unbreaks build & test on FreeBSD 9.1-STABLE
with GNU libiconv while it does not break build & test on Linux Mint
14 Nadia or Mac OS X 10.8.4.  If it breaks anything, let me know.