The name UBUNTU is misleading since it is only used to select the keyring backend (which is gnome keyring on linux).
This also allows to support other keyrings on linux in the future (e.g. KDEs keyring)
Although it's already quite ugly that we overwrite QMAKE_CC like this,
hardcoding 'g++' can break the build where prefixed versions of
the toolchain are expected and where QMAKE_CXX is already properly
defined. So we just use QMAKE_CXX variable here.
This fixes build failures on some distributions, e.g.
https://galileo.mailstation.de/jenkins/job/lipidity/36/console
