failed to load external entity "http://docbook.sourceforge.net/release/xsl/current/manpages/docbook
系统是：
guoyz@debian:~/mosquitto-master$ lsb_release -a
No LSB modules are available.
Distributor ID: Debian
Description: Debian GNU/Linux 9.9 (stretch)
Release: 9.9
Codename: stretch
编译mqtt的mosquitto服务器源码时，出现以下问题。
问题：
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"
compilation error: file manpage.xsl line 3 element import
xsl:import : unable to load http://docbook.sourceforge.net/release/xsl/current/manpages/docbook.xsl
compilation error: file libmosquitto.3.xml line 4 element refentry
xsltParseStylesheetProcess : document is not a stylesheet
Makefile:78: recipe for target 'libmosquitto.3' failed
make[1]: *** [libmosquitto.3] Error 5
make[1]: Leaving directory '/home/guoyz/mosquitto-master/man'
Makefile:50: recipe for target 'docs' failed
make: *** [docs] Error 2
解决办法：
guoyz@debian:~/mosquitto-master$ sudo apt-get install xmlto