New autoconf macro

Hi, I did some changes in aclocal.m4 and tex-site.el.in, mostly as
proof-of-concept.

Try this with

./configure --with-auto-dir=woozle
and
./configure --with-auto-dir=/woozle

and see the result in tex-site.el.  The Lisp removes possible double
slashes, and adds a single slash at the end.

AC_LISPIFY_DIR should be good for all such directories.

-- 
David Kastrup, Kriemhildstr. 15, 44793 Bochum