The AsciiDoc 7.1.2 configuration files included in the ELinks
source tree apparently aren't compatible with AsciiDoc 8.4.4:
[ASCIIDOC] doc/elinks.1.xml
FAILED: [listdef-bulleted] missing section: [listtags-None]
make[1]: *** [elinks.1.xml] Error 1
Fix this by including asciidoc.py from AsciiDoc 7.1.2 as well.
The build system now doesn't care at all whether the user has
installed some version of AsciiDoc or not.
