Port commit-msg to mawk

Problem reported by Ted Zlatanov in:
http://lists.gnu.org/archive/html/emacs-devel/2014-12/msg01093.html
* build-aux/git-hooks/commit-msg (space, non_space, non_print):
New vars.  Use them as approximations to POSIX bracket expressions,
on implementations like mawk that do not support POSIX regexps.