Build: Improve dependency on asciidoc

[digulla]

Either make the dependency to asciidoc more prominent (document it somewhere in the README and the site) or change the Makefile to skip the step silently when a2x is missing.

The way it is now, the build fails with an error when asciidoc isn't installed and the error message "a2x is missing" isn't very helpful - Google returns some weird results when I google for "a2x".

[klali]

this is currently mentioned at https://github.com/Yubico/yubico-pam/blob/master/README#L46 and only required when building from git.

[digulla]

Somehow, I missed that line README.

That said, I'm a big fan of "build works out of the box". Can you configure autoconf to "disable" the build of the docs when asciidoc isn't found?

[Mrten]

I'm also seeing this when building the .deb from the PPA. The first time everything is OK, but after a ./debian/rules clean I need a2x.