kde test fail in 16.04

[Harald Sitter]

<pitti> sitter, Riddell: several KDE tests now fail with "FAIL stderr:
cc1: warning: command line option ‘-std=c++11’ is valid for C++/ObjC++
but not for C"; e. g. kio, plasma-workspace, kcoreaddons
<pitti> these could be quiesced with "allow-stderr", but presumably
this should be fixed more properly?

Would be good if someone could take a look. If you have question I am
sure pitti can help.

HS