An oddity in GNU Make's definition of the MFLAGS and MAKEFLAGS variables

make-3.81 -rR -j3

early MFLAGS: -Rr
early MAKEFLAGS: Rr
late MFLAGS: -Rr --jobserver-fds=3,4 -j
late MAKEFLAGS: Rr --jobserver-fds=3,4 -j

Makefile

f := $(MFLAGS)
z := $(MAKEFLAGS)
all:
	@echo early MFLAGS: $f
	@echo early MAKEFLAGS: $z
	@echo late MFLAGS: ${MFLAGS}
	@echo late MAKEFLAGS: ${MAKEFLAGS}