Rework CMake plugin detection logic
Rework CMake plugin detection logic

The "continue()" command was not added until CMake v3.2, and we
want to support older versions at this time.

sutartmelson/girder,RafaelPalomar/girder,manthey/girder,Kitware/girder,msmolens/girder,adsorensen/girder,jbeezley/girder,essamjoubori/girder,Kitware/girder,Kitware/girder,data-exp-lab/girder,data-exp-lab/girder,msmolens/girder,manthey/girder,jbeezley/girder,msmolens/girder,Xarthisius/girder,RafaelPalomar/girder,data-exp-lab/girder,data-exp-lab/girder,kotfic/girder,essamjoubori/girder,sutartmelson/girder,adsorensen/girder,RafaelPalomar/girder,jbeezley/girder,Xarthisius/girder,salamb/girder,essamjoubori/girder,kotfic/girder,adsorensen/girder,salamb/girder,salamb/girder,adsorensen/girder,data-exp-lab/girder,manthey/girder,jbeezley/girder,kotfic/girder,RafaelPalomar/girder,msmolens/girder,essamjoubori/girder,salamb/girder,Xarthisius/girder,girder/girder,girder/girder,manthey/girder,Kitware/girder,sutartmelson/girder,sutartmelson/girder,kotfic/girder,msmolens/girder,girder/girder,girder/girder,RafaelPalomar/girder,Xarthisius/girder,kotfic/girder,sutartmelson/girder,adsorensen/girder,salamb/girder,Xarthisius/girder,essamjoubori/girder