Kris Thielemans authored Mar 04, 2017 and Brad King committed Mar 06, 2017

In the case that we're using `find_path` to find the include directory,
only the `_DIR` variable was set, while in all other cases only the
`_DIRS` variable was set.  Set the `_DIRS` variable in all cases (while
keeping `_DIR` as result of `find_path`).  Also document the `_DIRS`
variable.

Fixes: #16694