Pass QT_BINARY_DIR as HINTS to find_program(...)

When trying to locate xmlpatterns and qhelpgenerator QT_BINARY_DIR
should be passed as HINTS rather than PATHS. Otherwise, we can end
up with qmake being specified but system versions of xmlpatterns
and qhelpgenerator being used (as the standard system environment
variable PATH is searched before looking at PATHS), if these versions
don't match it can cause problems.

Change-Id: Id71bb4b95f117a347d9f004e37e1103aba71231a
Merge topic '14714-find_program-fix' into next

9f1f9df6 Pass QT_BINARY_DIR as HINTS to find_program(...)