Ability to read an optional configuration file
which is read if exists, we can even decide
which configuration file (default is config.mk) to use at
the command line like this:
$ make # it will read config.mk if exists
$ make CONFIG_FILE=my-badass-conf.mk
$ make CONFIG_FILE=no-retexturing.mk
As always, build variables still can be set as well by command line.
$ make WITH_SYSTEMWIDE=/media/cdrom/quake2-data CONFIG_FILE=my-badass-conf.mk
