build: make a2x utility optional

Only build manpages if the a2x utility is found.
This makes a missing a2x binary a warning instead of a fatal error.

Continuous integration builds can now optionally forgo install of
the huge list of dependencies for asciidoc, which will allow better
turnaround time for quick tests.
test: spellcheck man page sources instead of generated files

There is no reason that spell check has to be done on generated files
instead of their asciidoc sources. In case manpages are not built,
check source *.adoc files instead during 'make check' so that we still
catch spelling errors.
travis: do not install asciidoc during travis-ci build
Merge pull request #111 from grondo/a2x-optional

make manual page generation optional part of build