PHONON_NO_CAPTURE cmake feature

[Aleix Pol]

Hi,
I was trying to build phonon on some "unusual" platform (because
phonon is a KDE dependency and I need kdelibs). Since I wanted to get
it working fast I decided to enable the PHONON_NO_CAPTURE flag.

I saw that it was not compiling, so I figured out I'd fix it (like I
did with pulse, see my commits), then I realized that it can't be
fixed because of some method that returns a reference (see
mediasource.cpp:250).

Since the fix is not trivial anymore because of some decisions to be
made, I'll leave it up to the phonon crew.

Cheers!
Aleix

PS: it compiled, finally :)