FindQt4: Don't use Qt component _FOUND vars before they're defined (#14286)

This fixes a regression introduced by commit b8af46cb (FindQt4: Don't
fail if certain Qt modules are unavailable., 2013-05-23)

The _FOUND variable is set by the _QT4_ADJUST_LIB_VARS macro, so
invoke that unconditionally, and guard only the set_property calls,
as described in the original bug report (#14165).
Merge topic 'fix-FindQt4-available-modules' into next

969e7ee FindQt4: Don't use Qt component _FOUND vars before they're defined (#14286)
set_property: Do not remove a property when APPENDing nothing
Merge topic 'dev/property-append-with-empty-string' into next

31bd83e set_property: Do not remove a property when APPENDing nothing