The = assignment operator
make: "cmdline-undefined.mk" line 29: From the command line: Undefined is .
make: "cmdline-undefined.mk" line 30: From .MAKEFLAGS '=': Undefined is .
make: "cmdline-undefined.mk" line 31: From .MAKEFLAGS ':=': Undefined is .
make: "cmdline-undefined.mk" line 35: From the command line: Undefined is now defined.
make: "cmdline-undefined.mk" line 36: From .MAKEFLAGS '=': Undefined is now defined.
make: "cmdline-undefined.mk" line 37: From .MAKEFLAGS ':=': Undefined is now defined.

The := assignment operator
make: "cmdline-undefined.mk" line 29: From the command line: Undefined is .
make: "cmdline-undefined.mk" line 30: From .MAKEFLAGS '=': Undefined is .
make: "cmdline-undefined.mk" line 31: From .MAKEFLAGS ':=': Undefined is .
make: "cmdline-undefined.mk" line 35: From the command line: Undefined is now defined.
make: "cmdline-undefined.mk" line 36: From .MAKEFLAGS '=': Undefined is now defined.
make: "cmdline-undefined.mk" line 37: From .MAKEFLAGS ':=': Undefined is now defined.

exit status 0