autotools.eclass: dropping of M4DIR and explicit GNU m4 requirement
first up, i can't find anyone using M4DIR anymore (has been deprecated for a
while at this point), so i'm dropping it wrt AT_M4DIR.
next, i'm explicitly noting GNU m4 as a requirement in the autotools.eclass.
i believe this has always been de-facto case (we certainly use non-POSIX flags
like -I/--include), and i wanted to use a few more features without having to
worry about portability. plus, the autoconf website says GNU m4 is required