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.
Both my linux machine and my OSX machine don't have this command, so it may not be common enough to require. There are some snippets in #16552 which may be useful, however.