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

[David Faure]

dfaure accepted this revision.
This revision is now accepted and ready to land.

REPOSITORY
  R239 KDELibs4Support

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

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