Makefile keep track of variables

[potsrevennil]

This PR resolves the issue mentioned in https://github.com/pq-code-package/mlkem-c-aarch64/issues/75

[mkannwischer]

Yes, @potsrevennil.

I verified that now the Makefile triggers a re-build when a different CYCLES flag is passed.

Maybe also keep track of the CFLAGS that are potentially passed from the outside?

[potsrevennil]

@mkannwischer We cannot keep track of CFLAGS for now, since we chose to modify CFLAGS with environment variables (see discussion in https://github.com/pq-code-package/mlkem-c-aarch64/pull/69), and the current approach can only keep track of makefile variables.

Since the purpose of keep tracking of CFLAGS is that we want to avoid make clean while compiling for different architectures. There might be two ways of doing this:

1. Add a makefile variable like EXTRACFLAGS, and set -mcpu=... via this EXTRACFLAGS, and keep track of it.
2. Add a makefile variable like PLATFORM, and set mcpu, march flags accordingly, like mlkem-c-embedded have done

Maybe we can discuss this further in https://github.com/pq-code-package/mlkem-c-aarch64/issues/75, and open up a separate PR for solving this.

[mkannwischer]

Okay, understood.
Let's merge this one first, and think about the CFLAGS separately.
For now users using custom CFLAGS need to run make clean in between, but I guess that is fine for now.