Remove -lrt from LDFLAGS.

- This might be required on some embedded system, but from what I can see
  all the functions we use from real time extensions are included in libc
  and the program seems to work fine without it.
- It doesn't exist on MingwW or Mac, so I think it's better to remove it
  completely and add it later on any system that actually requires it.
- Related to #85.