Warnings from asciidoc on Fedora 29

On Fedora 29 with asciidoc 8.6.10, the build emits:

[ 52/167] Compiling docs/ntpd.txt
/usr/bin/asciidoc:471: DeprecationWarning: Flags not at the start of the 
expression '^(?u)[^\\W\\d][-\\w]*$'
  return re.match(r'^'+NAME_RE+r'$',s) is not None

External links:

• Reported to asciidoc: https://github.com/https://github.com/asciidoc/asciidoc-py3/issues/13
• Fixed upstream by asciidoc: https://github.com/https://github.com/asciidoc/asciidoc-py3/pull/14
• Reported to RedHat: https://bugzilla.redhat.com/show_bug.cgi?id=1655337

/cc @hal.murray