How does one modify the macro to use a different compiler in the makefile?

To modify the macro to use a different compiler in the makefile, you would need to edit the makefile directly. Options B and C are incorrect because changing the compiler installation path or using an environment variable would not directly modify the makefile. Option D is also incorrect because the makefile should not be compiled again, but rather edited directly to change the compiler. Therefore, the only correct option is to edit the makefile directly.