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 don't think we'll be able to rely on `CFG_LIBDIR_RELATIVE` solely for this purpose. Our nightly distributions are built with no `--libdir` or `--bindir` flag which means that they need to work on all systems they're installed on. This means that we need to detect at runtime whether to use lib/lib32/lib64.