Automatically handle setting make variables

[deciduously]

Our current builders allow setting environment variables before invoking make. Any variables set this way will become make variables if no other setting exists in the makefile. However, if the makefile sets the value of a variable, it will not inherit from the environment and instead only use that setting.

To override these, the desired value must be passed as an argument to make: make MYVAR="val", not MYVAR="val" make.

Provide a convenient way to construct make invocations which explicitly pass values as overrides in addition to setting them in the environment.