Remove dependency on docbook

[krichter722]

Docbook doesn't seem to be a well maintained software:

• The master branch of project https://github.com/docbook/docbook fails to build for months
• https://github.com/docbook/xslt10-stylesheets.git requires a ton of configurations before it builds locally
• https://sourceforge.net/p/docbook/ hasn't seen an update since 2015 (and has an outstanding bug preventing a successful build since 2010, see https://sourceforge.net/p/docbook/bugs/1143/ for details)

Furthermore the relation between all these projects and forks and relaunchs seem unclear and uncoordinated.

colord checks the return code of the command xsltproc --nonet http://docbook.sourceforge.net/release/xsl-ns/current/manpages/docbook.xsl which causes the build to fail if it doesn't return with 0. The file can easily be provided on popular OS by installing a system package (like docbook-xsl-ns on Ubuntu 19.04), however building from source is close to impossible due to the chaotic situation sketched above.

Maybe there's a change to migrate to a better maintained documentation build tool.

[hughsie]

In fwupd we use help2man -- would that be a better option going forward?

[krichter722]

Seems to be maintained and certainly easy to install (given the fact that it's standardized GNU software). I just filed a few issues with docbook. Their CI is broken and the build system is outdated. If that's fixed, I'll take a look at improvements which make the docbook build system reliable and standardized. It's a longer shot, but might save you the time to migrate to help2man.