Make optimization level configurable via CFLAGS env variable

[LiviaMedeiros]

Let's make CFLAGS="-Os" make possible.

[ValdikSS]

Well, it's kind of possible even without this change. When you set the variable inside make (make CFLAGS="-Os"), then the definition skips appending.
If you do that as an env (CFLAGS="-Os" make), it does not.

So you can just run `make CFLAGS="-Os -std=gnu99"

[LiviaMedeiros]

Sure, but it still should be agnostic to other flags. make CFLAGS="-Os" would override -std=gnu99 and make it impossible to set different flags in nested makefiles.