The relevant error in the log seems to be
:info:build xsltproc --novalid http://docbook.sourceforge.net/release/xsl/current/manpages/docbook.xsl dia.dbk >dia.1
:info:build http://docbook.sourceforge.net/release/xsl/current/manpages/docbook.xsl:1: parser error : Document is empty
:info:build
:info:build ^
:info:build http://docbook.sourceforge.net/release/xsl/current/manpages/docbook.xsl:1: parser error : Start tag expected, '<' not found
:info:build
:info:build ^
:info:build cannot parse http://docbook.sourceforge.net/release/xsl/current/manpages/docbook.xsl
:info:build make[3]: *** [dia.1] Error 4
Are you able to access http://docbook.sourceforge.net/release/xsl/current/manpages/docbook.xsl in your web browser?
