Compiling config.c on Mac OS X (and very likely other BSDs)

In order to compile bti, the symbols `strndup`, `getline`, `strchrnul` and `dprintf` are required.
However, these are missing on Mac OS X, as well as other BSD derivates.

This commit fixes at least `getline` and `strchrnul` (`strndup` and `dprintf` aren't annoying me, since I don't need url shortening).