bootstrap with MAKEFLAGS set

["Johnny C. Lam" <jlam%NetBSD.org@localhost>]

A user on IRC had problems running bootstrap which was tracked
down to having MAKEFLAGS set in the shell environment to some
value that bmake doesn't understand.

Should we modify the bootstrap script to throw an error if
MAKEFLAGS is set, and emit a suitably descriptive explanation
why?  We can always unilaterally unset MAKEFLAGS within the
bootstrap script, but the user is still likely to have have
problems when trying to build packages after the bootstrap is
complete.

Regards,
-- 
Johnny C. Lam
jlam%NetBSD.org@localhost