D9892: cmake: Use the official CMake variable for building as a static plugin.

[Marco Martin]

mart added a reviewer: lepagevalleeemmanuel.

REPOSITORY
  R169 Kirigami

REVISION DETAIL
  https://phabricator.kde.org/D9892

To: mart, #kirigami, lepagevalleeemmanuel
Cc: plasma-devel, apol, davidedmundson, mart, hein