This extension uses an expat compat layer by default. It can use also expat, which can be found at » http://www.jclark.com/xml/expat.html. The Makefile that comes with expat does not build a library by default, you can use this make rule for that:
libexpat.a: $(OBJS)
ar -rc $@ $(OBJS)
ranlib $@
A source RPM package of expat can be found at » http://sourceforge.net/projects/expat/.