Advantages of export:

1. Works out of the box and sets things up if we installed the library in a different path.
2. Can have macros and functions see Qt5DBusMacros.cmake from the Qt5DBus module for example.
3. Can also add targets for executable, see Qt5DBusConfigExtras.cmake from Qt5DBus.
4. Declare directory for public headers.

A use file doesn't have any real advantage over export files, how much could would it save?
We can always export targets with the same name for both Qt4Xdg and Qt5Xdg, users will just select which version:

```
if(USE_QT5)
  find_package(Qt5Xdg REQUIRED)
else
  find_package(Qt4Xdg REQUIRED)
endif()

and then depend on the QtXdg target.

I think there's no problem using export files for a Qt 4 library too.