Marking Solid's NetworkManager backend to compile only if NM-0.7 or

0.8 headers are installed. The NM-0.9 backend is in
git://anongit.kde.org/networkmanagement/solidcontrolfuture (nm09 branch)
now and is automatically compiled/installed when building Plasma NM.
When it is ready we are going to move the backend back to kde-workspace.
The networkmanager-0.7 backend supports and will only support 0.7
and 0.8 versions of NetworkManager.

BUG: 271289