Define environment variables

[EastAPOLO]

In fuf.c you get the value of environment variables several times by calling getenv. I think it would look more readable if they were defined as constants so you do not have to keep typing the function.

I can send this https://github.com/EastAPOLO/fuf/commit/0090fced59a67e127988d253cc54153739d9ac97 as a pull request if you want.

[Ckat]

not really seeing the point in making a define just because I do a function call multiple times, theres far more that I do exactly the same in multiple places. it really doesnt aid readability as it was already clear what I was doing