Add support for --bindir and remember the relative locations of bindir and libdir vs prefix (ie: sysroot)

Potential downsides:
 - right now adds a build dependency on `realpath`, which I'm not sure everyone has, to determine relative directories. It's possible we could substitute something else if this matters.

Upsides:
 - `--libdir` now avoids breaking things at runtime if it isn't in one of the expected locations
 - config diff between windows and everyone else is now confined to the configure script.
I'm not sure this is really an issue with putting all the libraries in `lib`. One of `lib32` or `lib64` will probably be a symlink to `lib`, and all of them (that exist and are in use on the system) will almost certainly be in ld.so's search path.

Do you know of a system where use of `lib32` or `lib64` over `lib` is required?