make manual page generation optional part of build

When creating build roots and OS images on the fly for testing, it was getting very cumbersome and time consuming to pull in asciidoc every time. Installing asciidoc and all its dependencies just for the `a2x` utility when you want to test actual code seems a bit of overkill.

This very small set of patches makes a missing `a2x` program a warning instead of an error, and disables the manual page generation unless `a2x` is present.

We still test spelling in mapages by using the `*.adoc` source instead of generated files, I can't really see any problem with that since I'm not sure how generating manpages could introduce spelling errors.

Finally, the travis-ci build script is updated so that asciidoc and its dependencies are not installed. This should speed up travis builds a little bit.

One issue with this change is that actual manpage generation won't be tested with travis. Perhaps there is some way to check the list of files changed, and install asciidoc if any manpages were added or changed. Or eventually commit 1f878adb5c060ea397c41caf145a43e27f188871 could be reverted.