autoconf: Use autoconf to write installation directories to config.mak.autogen

autoconf: Use autoconf to write installation directories to config.mak.autogen

This is beginning of patch series introducing installation configuration
using autoconf (and no other autotools) to git. The idea is to generate
config.mak.autogen using ./configure (generated from configure.ac by running
autoconf) from config.mak.in, so one can use autoconf as an _alternative_ to
ordinary Makefile, and creating one's own config.mak. Local settings in
config.mak override generated settings in config.mak.autogen

This patch includes minimal configure.ac and config.mak.in, so one can set
installation directories using autoconf generated ./configure script
e.g. ./configure --prefix=/usr

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