Fix xdev. Clang and libc were not building correctly on older worlds, for
example, on 9.1.
 * To fix clang add an _xb-bootstrap-tools target that mirrors the existing
   bootstrap-tools target in the full world.
 * For libc have the compiler use the newly installed includes, and, while
   here, tell the compiler about the xdev library path as some other
   libraries will link against the installed libraries.
Fix xdev-install when installing to a location other than /