Optional configuration file

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`