Define XML_STATIC in Expat header instead of Expat projects.

This reverts r74444 and defines XML_STATIC unconditionally in Expat sources
themselves as we always use the built-in Expat as a static library, so this
solution is simpler and more portable.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74446 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775