Fix linking errors when using wxXML in sttaic monolithic build: in this case
we must link with expat but it was listed too early, before wxWidgets
libraries using it, in the bakefile-generated makefiles.
Now add it to the end of the linker command line too by including -lexpat in
EXTRALIBS_FOR_BASE which are appended to the linker arguments.
Notice that now -lexpat is linked twice because it is still added by
<wx-lib>xml</wx-lib> in bakefiles, even in monolithic build, but I don't know
how to avoid it and it is harmless anyhow.