Made some changes to make DOOM work on Ubuntu

Hi, I'm a newbie to open source. I took this as an exercise to make DOOM run on Ubuntu.
I found a couple of issues. Running 'make' as is was failing. Done some changes to Makefile and some of the sources to make it build.

Also when I ran DOOM within the Xephyr window in Pseudocolor 8bpp mode, the colormap wasn't being picked up. Made a change to correct this.