Update the version of Expat used to 2.1.0.

Merge with the latest version, mostly discarding our changes to 1.95.6 as they
seem to be unnecessary any longer, keep just bakefile-specific addition to
configure.

Also update our own makefile to define HAVE_EXPAT_CONFIG_H which is now needed
by Unix build.

Closes #11677.


git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74436 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775