Ordinarily variable assignments in Makefiles are done simply with an unconditional assignment: the equals sign. There are variants of this:

FOO := $(BAR)   Avoid recursively expand variables.
FOO ?= bar      Assign only if $(FOO) is not already set.
FOO += bar      Add more to a variable.
FOO != bar      Execute a shell script and assign result to $(FOO).

See also:

• https://www.gnu.org/software/make/manual/html_node/Conditional-Example.html
• https://www.gnu.org/software/make/manual/html_node/Flavors.html

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew style with your changes locally?
• Have you successfully run brew tests with your changes locally?

Not sure what new tests would apply or be required for my changes.
Running brew tests showed failures not caused by my changes.