Fix building the WMF plugin

9df63fd6

 made it possible to build only the audio decoder service from
the WMF plugin. When that change was merged to 5.8 it was not ported
to the new configuration system.
Added a new '-mediaplayer-backend' configure option to select the media
player backend to use on Windows. '-wmf-backend' is kept for backward
compatibility.

Change-Id: I902b2e4147f9a05b92118152020afc2638bf41e1
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@qt.io>