I am able to get this to build locally, but trying to distinguish what is different compared with a clean system.
Here is the listing of files contained in the xhtml package:
xhtml-1.0.20020801_4:
/usr/local/share/xml/dtd/xhtml/catalog.xml
/usr/local/share/xml/dtd/xhtml/xhtml-dcl.soc
/usr/local/share/xml/dtd/xhtml/xhtml-lat1.ent
/usr/local/share/xml/dtd/xhtml/xhtml-special.ent
/usr/local/share/xml/dtd/xhtml/xhtml-symbol.ent
/usr/local/share/xml/dtd/xhtml/xhtml.soc
/usr/local/share/xml/dtd/xhtml/xhtml1-frameset.dtd
/usr/local/share/xml/dtd/xhtml/xhtml1-strict.dtd
/usr/local/share/xml/dtd/xhtml/xhtml1-transitional.dtd
/usr/local/share/xml/dtd/xhtml/xhtml1.dcl