curious behaviour of $MAKEFLAGS and leading hyphen

[Robert P. J. Day]

  from some time back, when i first learned about the MAKEFLAGS
variable, i was always a bit put off that (for historical reasons) it
insisted on representing the command-line options *without* the
leading hyphen.  so if i called make with

  $ make -I /tmp

MAKEFLAGS would, when calling a subshell, be set to "I /tmp" and, i'm
assuming, the callee would then use "-${MAKEFLAGS}", right?

  but how to explain "make -j 5" then and what it does with MAKEFLAGS?

        MAKEFLAGS =  --jobserver-fds=3,4 -j

???.  this seems inconsistent with the normal MAKEFLAGS behaviour WRT
to that leading hyphen.  thoughts?

rday