Merge topic 'cmake-ignore-prefix-path'

201d8c4298 find_*(): Add CMAKE_IGNORE_PREFIX_PATH variable
bd805a51ae Refactor: Keep track of prefixes in cmSearchPath

Acked-by: Kitware Robot <kwrobot@kitware.com>
Tested-by: buildbot <buildbot@kitware.com>
Merge-request: !6880