(Suggestion) GNUmakefile: Change target "all" to build NetDuke32 by default
There was more than one instance (including last night) in which I forgot to pass the right target name to the command "make", thus getting mainline EDuke32 built instead. I also know of at least one individual who simply didn't know, was new to building NetDuke32 from the sources, and got EDuke32 instead.
Modifying the target "all" will also be more consistent with the NBlood repository, in which the target "all" was changed to cover Rednukem, NBlood and PCExhumed.