Plasma does not build without kdepimlibs

[Stephen Kelly]

Hi,

9edc8d34b3b0f9983f0eb014f8fbf4bcfcffc3f1 introduced a dependency in plasma 
on kdepimlibs for gpgme++.

The cmake check in kdelibs for kdepimlibs claims it is optional, but the 
build fails later. The stuff that uses gpgme++ should be compiled 
conditionally based on whether kdepimlibs was found.

Thanks,

Steve.