Use setup makefile + make by default to build libraries

Use setup makefile + make by default to build libraries

The advantages of this are
  (a) it's a step closer to getting -j working again (make -j works in
      an individual library, but not in libraries/ yet).
  (b) it's easier to hack on libraries: make dist/build/Foo.o
  (c) it's a step closer to getting HC bootstrapping again

The build system creates <lib>/GNUmakefile as part of 'make boot'.
This was chosen so as not to interfere with existing Makefiles, but
it's a bit of a hack.  (previously I used CabalMakefile, but that
means adding -f CabalMakefile each time you run make, and that's a
pain).