Add systemd unit files

Signed-off-by: Jonathan McCrohan <jmccrohan@gmail.com>
Allow build flags to be overridden by make process

Required to enable Debian buildi hardening:
https://wiki.debian.org/Hardening

Signed-off-by: Jonathan McCrohan <jmccrohan@gmail.com>
Merge pull request #28 from jmccrohan/master

Fixes discovered during Debian packaging of nyancat 1.4.1