[CMake] Path to installed target

[Alexey Livshits]

> Thing is, you can't *rely* on DESTDIR *not* being used by the user of
> your project. If a user shoots himself in the foot, that's his fault,
> but when designing your test architecture you IMHO shouldn't make the
> use of DESTDIR impossible. I'm sure Debian/Ubuntu/the/whole/zoo would
> object to that...

That's right. I don't want to rely on DESTDIR. That's the problem, not
a solution. Otherwise I could use CMAKE_INSTALL_PREFIX and I would be
fine.
-- 
BG,
Alexey