Simplified Makefile

I removed many cases that are not in practice useful. In particular,
it won't be anymore possible to:
- Link statically PLUMED if both "ar" and "ld -ro" do not work correctly.
- Link runtime PLUMED is "ar" does not work correctly.

Both statements are true only for installed PLUMED. Not installed plumed
still uses object files.

(see #352)