add helper targets and variables
use `gunzip -c` instead of `zcat` which only supports .Z and not .gz on Mac OS X
http://ml.nec-labs.com/download/data/mnist8m is dead, use infimnist instead
netcat is sometimes called nc
update autogen dependencies
Merge pull request #404 from sam-s/master

improve Mac OS X support: zcat cannot hangle *.gz