Add qjson_ variants in addition to QJSON_ ones

It is a sad world in which we live in and we ship a FindQJSon.cmake
that defines QJSON_ but it seems that some debian derived distros
are including a qjson-config.cmake that is not in 0.7.1 upstream (latest released version)
and that defines the qjson_ ones, so i guess we need both for people using
the qjson-config.cmake and for people using the FindQJSon.cmake

Buildsystem guys, is there a better solution?

CCMAIL: kde-buildsystem@kde.org