Copy description of build configuration variables from the commentary
in the top Makefile (from 'next' branch) to configure.ac, splitting
them into "autoconf" sections.

This is to be able to easily check which build/install configuration
variables are covered by current configure.ac
Signed-off-by: NJakub Narebski <jnareb@gmail.com>
Signed-off-by: NJunio C Hamano <junkio@cox.net>