Support NetworkManager 1.20 and do actually compile the NM backend

NetworkManager.pc and libnm-util.pc are no longer available in 1.20. The only header that kdelibs4support is using is NetworkManager.h, which is found in the include path of libnm.

Also make this actually work. The code was guarded by a never-defined if(NM_0_7)

Differential Revision: https://phabricator.kde.org/D24020