- Consider .hc files as real sources unless they are derived from
  .hs/.lhs sources (using the existing "derived sources" mechanism").

- Set PRE_SRCS using := rather than =, to avoid re-expanding the
  $(wildcard) multiple times.  In a simple test I did, this can save
  a 1/4 second (depending on the size of the directores involved) per
  make!