Find locally installed software first

This commit re-orders the search path prefix list from

  /
  /usr
  /usr/local

to

  /usr/local
  /
  /usr

so that locally-installed software is preferred.

This makes the search consistent with the Filesystem Hierarchy Standard:

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

See issue #9657.

Modules/Platform/UnixPaths.cmake

@@ -9,7 +9,7 @@ GET_FILENAME_COMPONENT(_CMAKE_INSTALL_DIR "${_CMAKE_INSTALL_DIR}" PATH)
 # search types.
 LIST(APPEND CMAKE_SYSTEM_PREFIX_PATH
   # Standard
-  / /usr /usr/local
+  /usr/local / /usr
 
   # CMake install location
   "${_CMAKE_INSTALL_DIR}"