Commit "Find locally installed software first" made /usr/local the first
prefix searched to be consistent with the Filesystem Hierarchy Standard:

  http://www.pathname.com/fhs/

The standard also implies that the root prefix "/" should not have any
package or development files.  The "/bin" and "/lib" directories should
have only minimal contents to boot the system.  No "/include" ever
exists.  This commit re-orders the search path prefix list from

  /usr/local
  /
  /usr

to

  /usr/local
  /usr
  /

to prefer package and development files over low-level system files.
See issue #10136.

On Cygwin /usr/lib == /lib and /usr/bin == /bin.  This change also makes
search results report locations as "/usr/..." instead of "/lib/...".
See issue #10122.