Pass linker flags to CMake.
Add, rather than replace, CFLAGS.
Add 'make scrub' target.

This is like 'make clean', only it works much harder and does a more thorough job. Necessary when you get into a funny CMake tangle.
Add a BSD makefile to drive GNU make.

Let people use regular 'make'; when we need GNU-isms (as we do in this directory, at least for now), call gmake under the hood.