maint: make sc_dd_O_FLAGS more robust

maint: make sc_dd_O_FLAGS more robust

The construct "diff ... || diff=1 || diff=" does not set the variable
in all cases.  This could be triggered with:

  $ env make diff=1 make sc_dd_O_FLAGS
  dd_O_FLAGS
  maint.mk: ./src/dd.c has inconsistent O_ flag lists
  cfg.mk:59: recipe for target 'sc_dd_O_FLAGS' failed
  make: *** [sc_dd_O_FLAGS] Error 1

* cfg.mk (sc_dd_O_FLAGS): Remember $? of the diff command directly
and check its value later rather than using the  above mentioned
mapping.