Better error message if one forgets to set KDEDIR variable

[Stephen Kelly]

Alexander Neundorf wrote:
>> > After resetting KDEDIR to the correct directory everything worked fine
>> > but dfaure told me to write this to the buildsystem list and thus Alex
>> > to mention that there should be a better error message for this
>> > problem.
> 
> 
> Fix pushed.


Thanks for that.