The = assignment operator
make: cmdline-undefined.mk:41: From the command line: Undefined is .
make: cmdline-undefined.mk:42: From .MAKEFLAGS '=': Undefined is .
make: cmdline-undefined.mk:43: From .MAKEFLAGS ':=': Undefined is .
make: cmdline-undefined.mk:47: From the command line: Undefined is now defined.
make: cmdline-undefined.mk:48: From .MAKEFLAGS '=': Undefined is now defined.
make: cmdline-undefined.mk:49: From .MAKEFLAGS ':=': Undefined is now defined.

The := assignment operator
make: cmdline-undefined.mk:41: From the command line: Undefined is .
make: cmdline-undefined.mk:42: From .MAKEFLAGS '=': Undefined is .
make: cmdline-undefined.mk:43: From .MAKEFLAGS ':=': Undefined is .
make: cmdline-undefined.mk:47: From the command line: Undefined is now defined.
make: cmdline-undefined.mk:48: From .MAKEFLAGS '=': Undefined is now defined.
make: cmdline-undefined.mk:49: From .MAKEFLAGS ':=': Undefined is now defined.

exit status 0