Cleaner generation of test vectors.

Cleaner generation of test vectors.

The makefile that generates the test vectors directly puts the
vectors.h header in the dist/tests/ directory.  No more weird script
to do half that work.

As a side effect, BSD users can now switch to gmake more easily (they
don't depend on a script to do stuff like `make || gmake`.

Closes #33, though not satisfactorily (the makefiles still rely on GNU
make, because portable makefiles are just crippled).