A makefile is a file that is included by most source packages and tells the compiles what to compile and how to compile it(with what flags)
Most of the time the Makefile is working unless you have your binaries somewhere else.
If you must edit it i suggest you learn something about coding C since most C programs use Makefiles.
Maybe there is not much to edit about the makefile that came with ZDoom.. you should try just to make and make install it.
if it fails searchs on some coding forms or take a look at the makefile.
btw the makefile is a file called "Makefile" also with most software you need to run a ./configure to make a config file for the compiler.
Maybe you are in luck and ZDoom works out of the box