The = assignment operator
make: cmdline-undefined.mk:31: From the command line: Undefined is .
make: cmdline-undefined.mk:34: From .MAKEFLAGS '=': Undefined is .
make: cmdline-undefined.mk:37: From .MAKEFLAGS ':=': Undefined is .
make: cmdline-undefined.mk:43: From the command line: Undefined is now defined.
make: cmdline-undefined.mk:46: From .MAKEFLAGS '=': Undefined is now defined.
make: cmdline-undefined.mk:49: From .MAKEFLAGS ':=': Undefined is now defined.

The := assignment operator
make: cmdline-undefined.mk:31: From the command line: Undefined is .
make: cmdline-undefined.mk:34: From .MAKEFLAGS '=': Undefined is .
make: cmdline-undefined.mk:37: From .MAKEFLAGS ':=': Undefined is .
make: cmdline-undefined.mk:43: From the command line: Undefined is now defined.
make: cmdline-undefined.mk:46: From .MAKEFLAGS '=': Undefined is now defined.
make: cmdline-undefined.mk:49: From .MAKEFLAGS ':=': Undefined is now defined.

exit status 0