This extension requires the libxml PHP extension. This means that passing in --enable-libxml is also required, although this is implicitly accomplished because libxml is enabled by default.
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 $@