Fix link error on Cygwin.
Patch partially by Michael Haubenwallner at
https://gitlab.gnome.org/GNOME/libxml2/commit/c65c9e8ee07e2dab0647392c2bd1795a5bc99829
* gnulib-local/lib/libxml/xmlexports.in.h: Don't assume that _WIN32 is defined
on Cygwin.
(XMLPUBVAR): Define as 'extern', not as empty.
