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

[codyps]

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.