Update makefile for Assignment 4

Be sure to include new memory environment:

Okay, memory size. I will set an environment var called RAM_MEM before running your run target . It is quite simple to read an environment variable in a makefile:

 foo :
    echo $$RAM_MEM
You can test it like this:

 $ RAM_MEM=300 make foo
, or, if you add it to your environment, remember to export it. Please don't set it in your makefile.
