Add configurable parameters

[ark0f]

No description provided.

[alexcrichton]

Thanks! I'll be honest in that I don't really know what these parameters are. Can you explain a bit more your rationale for sending this PR? The PR description is blank and the commit carries no information other than what the PR title already has, and I'm not really equipped to reverse-engineer what's going on here.

Also, are you sure this is safe from a safe Rust point of view? Some of the descriptions of these variables and usages internally looks like the constructor probably wants to be unsafe because it seems like things could behave badly if this is misconfigured.

And finally, in C if these are #defines do you know how it affects runtime performance if these are runtime-defined values? If that's a problem should they be associated constants perhaps?