Gracefully handle build configurations on systems without AsciiDoc

On systems without AsciiDoc the build configuration aborts telling users to pass --disable-asciidoc. This requires users to restart the build configuration which is annoying.

Instead the build configuration should handle these cases gracefully and show a message without aborting the configuration. In these cases i would also show a less verbose message and change it to something similar to systems without Python, see https://gitweb.torproject.org/tor.git/tree/configure.ac?id=4098bfa26073551fe3f525ada7fc9079a49fd4bb#n218.