Limit testing for an up-to-date version of make(1) to the world and

Limit testing for an up-to-date version of make(1) to the world and
buildworld targets by default, but allow it to be done for all user
targets by introducing a boolean option, named ALWAYS_CHECK_MAKE.

This change is by no means perfect and I don't even want to claim
this to be a solution. It does however address the fact that not
everybody likes to see make(1) rebuilt simply because the regression
test failed for some reason or other, including pilot error. It
therefore serves the purpose of keeping the crowd happy until we
have something better or simply reached a compromise.

The reasons for changing the default behaviour are:
o  It avoids a negative, possibly non-intuitive option,
o  It's according to POLA and fond of feet,
o  Only buildworld is documented to do its best to be
   successful at reasonably cost.

Reviewed by: gad, imp, obrien, peter