Qt conditional version check

[Will Stephenson]

What's the canonical way to enable compilation of a subdirectory depending on 
the version of Qt found?  

I want to be able to only compile the Network Management plasmoid if Qt 4.6 is 
found, but grepping svn yields no good examples, some people are doing manual 
comparisons of QT_MAJOR_VERSION, QT_MINOR_VERSION etc.

I'll document this on techbase.

cheers

Will