expat: Build without docbook
This fixes following build error on Debian 9:
configure: error: Your local docbook2man was found to work with SGML rather
than XML. Please install docbook2X and use variable DOCBOOK_TO_MAN to point
configure to command docbook2x-man of docbook2X.
Or use DOCBOOK_TO_MAN="xmlto man --skip-validation" if you have xmlto around.
You can also configure using --without-docbook if you can do without a man
page for xmlwf.