fixing always-dirty ki18n_install(po)

[Milian Wolff]

Hey all,

see https://bugs.kde.org/show_bug.cgi?id=460245 for some details.

Does anyone know how we could fix `ki18n_install(po)` to not mark the build 
dir as always-dirty? It's pretty annoying to have to wait for multiple seconds 
for the translations, even though nothing has changed at all.

The culprit is, as usual in such situations, the usage of `add_custom_target`:

```cmake
add_custom_target(pofiles-${pathmd5} ALL
...
add_custom_target(tsfiles-${pathmd5} ALL
```

Now, the issue is that `add_custom_target` cannot ever become clean, it is 
inherently broken by design in that regard. Usually `add_custom_command` is 
better for that reason, but that one has a different design deficiency: It 
does not support the `ALL` dependency...

So, does anyone know how to resolve this catch-22?

I only see two solutions:

a) fix cmake upstream to allow `ALL` for `add_custom_command`
b) change the i18n macro to take a target, e.g.:

```
ki18n_install(po TARGET kdevelop)
```

Then if that is given, use `add_custom_command`, otherwise keep the always 
dirty `add_custom_target` like before?

I would appreciate any input on that front,

thanks!
--  
Milian Wolff
http://milianw.de