- store the used compiler plus flags in the .%.dep files too and

- store the used compiler plus flags in the .%.dep files too and
  use this info to decide whether or not to rebuild something (if e.g. the
  filestamp of the prereq did not change but the cc or CFLAGS did).

  For files that we did not yet build we have no flags on record, so those
  did change inherently and we rebuild.

  Shouldn't be much slower than before.