Allow disabling adding the install prefix to the prefix search path.

In certain scenarios, it is preferable to keep a 'dirty' install prefix
than to clear it, and to expect that content will not be found there.
Add a CMAKE_FIND_NO_INSTALL_PREFIX variable that can be set to disable
searching the install prefix.
Merge topic 'add-CMAKE_FIND_NO_INSTALL_PREFIX' into next

fe057ab Allow disabling adding the install prefix to the prefix search path.