
In order to better support Xcode-only systems, where X11 libs and executables live under /usr/X11 but headers live in the SDK, move the x11_* helper methods into a new module. This allows us to keep some of the CLT/Xcode-only and Apple X11/XQuartz logic hidden from outside code, like ENV.x11. Since Apple's X11 is actually XQuartz, name the module "MacOS::XQuartz".