Use built-in linking rules (with its own variables).

Featuring familiar variables with widely available [documentation](http://www.gnu.org/software/make/manual/html_node/Implicit-Variables.html)! It's not totally done right (```-L``` stuff belongs in ```LDFLAGS```, ```-l``` in ```LDLIBS```, this just puts everything in ```LDLIBS```), but I'm going for a minimal change for now...