Deprecate --enable-windows to configure and replace with WINSUPPORT=0/1

in the defs file, so that it is easier to export a default value.
The local defs-default turns it on for all of us, while the default in
configure.in is off. It is currently off by default in defs-example.

I have left the --enable-windows and --disable-windows arguments for
backwards compatability; they just override whatever the defs file
says but otherwise have no effect if not specified.