macOS Mojave - build fails with: validity error: Could not load the external subset

[rstammer]

When trying to compile all submodules under macOS Mojave 10.14.6,
the build command stops with the following error:

validity error : Could not load the external subset "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd"

./mkman zauth.txt
Generating zauth.txt...
asciidoc -d manpage -b docbook -f  ./asciidoc.conf \
		-aczmq_version=2.0.1 -ozauth.xml zauth.txt
xmlto man zauth.xml
xmlto: /Users/robin/projects/rbczmq/ext/czmq/doc/zauth.xml does not validate (status 3)
xmlto: Fix document syntax or use --skip-validation option
I/O error : Attempt to load network entity http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd
/Users/robin/projects/rbczmq/ext/czmq/doc/zauth.xml:2: warning: failed to load external entity "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd"
D DocBook XML V4.5//EN" "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd"
                                                                               ^
I/O error : Attempt to load network entity http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd
warning: failed to load external entity "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd"
validity error : Could not load the external subset "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd"
Document /Users/robin/projects/rbczmq/ext/czmq/doc/zauth.xml does not validate
make[1]: *** [zauth.3] Error 13
rm zauth.xml
make: *** [all-recursive] Error 1
CZMQ compile error!
*** ../../../../ext/rbczmq/extconf.rb failed ***
Could not create Makefile due to some reason, probably lack of necessary
libraries and/or headers.  Check the mkmf.log file for more details.  You may
need configuration options.

Provided configuration options:
	--with-opt-dir
	--without-opt-dir
	--with-opt-include
	--without-opt-include=${opt-dir}/include
	--with-opt-lib
	--without-opt-lib=${opt-dir}/lib
	--with-make-prog
	--without-make-prog
	--srcdir=../../../../ext/rbczmq
	--curdir
	--ruby=/Users/robin/.rvm/rubies/ruby-2.1.2/bin/ruby
	--with-system-libs
	--without-system-libs
	--with-system-libs
	--without-system-libs
	--with-system-libs
	--without-system-libs
rake aborted!