D24020: Support NetworkManager 1.20 and do actually compile the NM backend

[Antonio Rojas]

arojas added a reviewer: Frameworks.

REPOSITORY
  R239 KDELibs4Support

REVISION DETAIL
  https://phabricator.kde.org/D24020

To: arojas, #frameworks
Cc: lbeltrame, kde-frameworks-devel, LeGast00n, GB_2, michaelh, ngraham, bruns