as per the discussion on kde-core-devel, add a KDE_SESSION_VERSION env var that holds the value of the major # of the running desktop, allowing one to run kde${KDE_SESSION_VERSION}-config and get the proper kde?-config.

i considered using cmake's configure_file to process this and add the 4 directly, but since this is only getting changed every 5 years or so and it would mean adding another cmake variable to the build, etc... i took the simple way out.

if there are objections to this being managed "by hand" i'll try and find the energy to automate it.