The rules only require that you have an ability score that meets the prerequisite, it does not put any requirements on how you get that ability score.
Using common sense, I would only allow reasonably permanent ability increases allow you to qualify for feats. By reasonably permanent, I mean things that can effectively give you the bonus 24/7. This is usually a magic item (ignoring the difficulty of sleeping with some of the items on, since it doesn't come into play for spellcasters getting bonus spell for enhanced spellcasting stats).
The rules do state that characters undergo training or studying between adventures (PHB, page 58). This happens "off camera" but does indicate that leveling up requires time on the part of the character, and is not instantaneous.
As such, I would not allow a short duration spell (such as fox's cunning) to help qualify for a feat or prc, since it doesn't last long enough to give a practical benefit for training purposes.
I would probably allow a druid to use their wild-shaped form stats to qualify, once they reach a level where they can choose to stay wild-shaped all or most of the day.
If you use an item to help qualify for a feat, and you later lose the use of the item, you still have the feat but cannot use it. Any other feats that use the feat as a prerequisite still work, as you still have the feat (most of the time this is a moot point as the other feats will usually have the same stat requirement).