autoconf: Preparing the way for autodetection

autoconf: Preparing the way for autodetection

Prepares configure.ac to output autodetected and selected (by using
--with/--without and --enable/disable parameters to generated
./configure script) building configuration in "git style", i.e. by
appending appropriate variables to output file config.mak.autogen
(via temporary file config.mak.append).

Signed-off-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>