add general Makefile

useful as a template for new projects, or *the* Makefile for a simple
program. easily configured & adapted. still kinda in progress, but it's
well commented.
do my homework :))) ('Homework 1', simple calc)

took me fuckin 45 mins lol
still, good fun getting C to do silly things. even used a case
statement, which was a nice refresher. mostly works -- only other thing
I'd especially like is being able to choose how many decimal places you
want when dividing. I know where I'd place it, but I don't know how I'd
actually use the input I get (something like an eval???).