asciidoc > 8.6.0 is a build requirement

[Jason Azze]

I've researched the documentation build failure on CentOS 6. It looks
like the version of asciidoc (and a2x) is the culprit.

CentOS 6.8 has asciidoc version 8.4.5, which includes a2x version 1.0.0.

The build on stock CentOS 6.8 fails with:

asciidoc: FAILED: ntpd-man.txt.man-tmp: line 6: section title expected

CentOS 7.2 has asciidoc 8.6.8 (and a2x 8.6.8), and the build works fine.

I started to try every released asciidoc version between 8.4.5 and
8.6.8 from https://github.com/asciidoc/asciidoc/releases until I found
the version where the error disappeared. 8.6.0 is the winner.

Here's the command I ran for my analysis. (Note that I shortened the
paths to asciidoc.conf and ntpd-man.txt.man-tmp below because they
only make sense on my test machine.)

/opt/asciidoc-alts/asciidoc-8.6.0/a2x.py --format manpage
--asciidoc-opts=--conf-file=asciidoc.conf --no-xmllint
ntpd-man.txt.man-tmp

I also discovered that version 8.5.2 of asciidoc works when I use this command:

/opt/asciidoc-alts/asciidoc-8.5.2/asciidoc.py --doctype=manpage
--conf-file=asciidoc.conf -d manpage -b docbook
"/ntpd-man.txt.man-tmp"