catkin_FOUND variable was never set

Shouldn't this be
```
  set(catkin_FOUND 1)
```
instead of
```
  set(catkin_FOUND)
```
?

From http://www.cmake.org/cmake/help/v2.8.3/cmake.html#command:set:
> If <value> is not specified then the variable is removed instead of set. See also: the unset() command.

Seems that `catkin_FOUND` was never set since the beginning of time...