building: Invoke `missing' on missing help2man

This gives the standard `missing' blurb for help2man if someone tries to
build manpages without having help2man installed, which is better (as
in, more helpful) than "help2man: command not found", in my opinion.

Signed-off-by: Anton Lofgren <alofgren@op5.com>
Merge pull request #57 from catharsis/configure-help2man

building: Invoke `missing' on missing help2man