Missing dependency in configure.ac

Just installed dev-tools on a fresh system and noticed few missing dependencies:

schwinn@schwinn-gsi:~/git/xfce4-dev-tools$ make -j4
Making all in docs
make[1]: Entering directory '/home/schwinn/git/xfce4-dev-tools/docs'
xsltproc --nonet http://docbook.sourceforge.net/release/xsl/current/manpages/docbook.xsl xdt-csource.xml
/bin/bash: line 1: xsltproc: command not found
make[1]: *** [Makefile:535: xdt-csource.1] Error 127
make[1]: Leaving directory '/home/schwinn/git/xfce4-dev-tools/docs'
make: *** [Makefile:406: all-recursive] Error 1
schwinn@schwinn-gsi:~/git/xfce4-dev-tools$ sudo apt-get install xsltproc

schwinn@schwinn-gsi:~/git/xfce4-dev-tools$ make -j4
Making all in docs
make[1]: Entering directory '/home/schwinn/git/xfce4-dev-tools/docs'
xsltproc --nonet http://docbook.sourceforge.net/release/xsl/current/manpages/docbook.xsl xdt-csource.xml
I/O error : Attempt to load network entity http://docbook.sourceforge.net/release/xsl/current/manpages/docbook.xsl
warning: failed to load external entity "http://docbook.sourceforge.net/release/xsl/current/manpages/docbook.xsl"
cannot parse http://docbook.sourceforge.net/release/xsl/current/manpages/docbook.xsl
make[1]: *** [Makefile:535: xdt-csource.1] Error 4
make[1]: Leaving directory '/home/schwinn/git/xfce4-dev-tools/docs'
make: *** [Makefile:406: all-recursive] Error 1
...
<some web search and trial + error happened here>
...
schwinn@schwinn-gsi:~/git/xfce4-dev-tools$ sudo apt-get install docbook-xsl

--> build runs fine

... will open a MR for it