Idea: Failing the build unless you explicitely chose which Okular UI you want to build

[Albert Astals Cid]

Right now we build by default both the Desktop UI and the Mobile UI.

In my opinion, for the user, that doesn't make sense, you either want one installed or the other installed, but not both.

On the other hand, it makes lots of sense for CI and for developers, because you don't want to break the build of one while working on the other.

My suggestion is to fail the build unless you specify which of the UI you want.

So cmake . would fail and tell you something like:

    Please specify -DOKULAR_UI=Desktop|Mobile|Both

And then users/distros would only build the one they want, and we devels/CI can still chose Both.

What do you think?

Cheers,
  Albert