Don't assume that all UNIX platforms have <iconv.h> - explicitly check

Don't assume that all UNIX platforms have <iconv.h> - explicitly check
whether it's present.

This puts HAVE_ICONV_H in the "config.h" file, so put in an entry for it
in "config.h.win32" - an entry that doesn't define it - and get rid of
the check for WIN32 in "xmlstub.h".

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@4124 f5534014-38df-0310-8fa8-9805f1628bb7