Add a way to silence stderr logging

[spenczar]

I'm linking to ASSIST as a library. I have my own ways I'd like to handle errors and provide warnings to the user. It'd be nice if I could disable the fprintf(stderr, ...) calls in ASSIST, maybe with a -D flag during compilation or something.

I don't really feel that I need to be able to recover those messages in any way; just suppressing them is fine, especially if there are well-defined error codes, which is mostly the case throughout ASSIST.

[hannorein]

Fair enough. I don't think there's any harm done by adding a few #ifdefs. Doing anything more complicated seems like an overkill.

(Probably not what you want, but you won't see any errors after calling fclose(stderr). )