Description of problem:
When running gtk-doc in a package build (exo) the build fails with: 

cd ./html && gtkdoc-mkhtml exo ../exo-docs.sgml
file:///usr/share/sgml/docbook/xsl-stylesheets-1.72.0-1.fc7/html/autoidx.xsl:4:
warning: failed to load external entity
"file:///usr/share/sgml/docbook/xsl-stylesheets-1.72.0-1.fc7/common/entities.ent"
%common.entities;
                 ^
Entity: line 1:
 %common.entities;
                  ^
file:///usr/share/sgml/docbook/xsl-stylesheets-1.72.0-1.fc7/html/autoidx.xsl:32:
parser error : Entity 'primary' not defined

Version-Release number of selected component (if applicable):
1.72.0-1.fc7

How reproducible:
Try and use gtk-doc to build html docs. 

The: 

/usr/share/sgml/docbook/xsl-stylesheets-1.72.0-1.fc7/common/entities.ent file is
missing. It's there on the docbook site, just not in the package. 

The autoidx.xsl file includes it: 

<?xml version="1.0"?>
<!DOCTYPE xsl:stylesheet [
<!ENTITY % common.entities SYSTEM "../common/entities.ent">
%common.entities;
]>