BUG: Ensure the targets file is only included once.
BUG: Ensure the targets file is only included once.

The vtkCommon target should always exist for VTK, and so makes a good
equivalent to a header guard. Projects like Titan need this as we make
multiple find_package(VTK) calls.

jmerkow/VTK,daviddoria/PointGraphsPhase1,berendkleinhaneveld/VTK,SimVascular/VTK,jmerkow/VTK,spthaolt/VTK,gram526/VTK,demarle/VTK,demarle/VTK,cjh1/VTK,keithroe/vtkoptix,gram526/VTK,hendradarwin/VTK,daviddoria/PointGraphsPhase1,SimVascular/VTK,sankhesh/VTK,SimVascular/VTK,ashray/VTK-EVM,demarle/VTK,collects/VTK,spthaolt/VTK,mspark93/VTK,SimVascular/VTK,spthaolt/VTK,msmolens/VTK,spthaolt/VTK,berendkleinhaneveld/VTK,johnkit/vtk-dev,sumedhasingla/VTK,demarle/VTK,msmolens/VTK,hendradarwin/VTK,candy7393/VTK,cjh1/VTK,johnkit/vtk-dev,ashray/VTK-EVM,sumedhasingla/VTK,SimVascular/VTK,candy7393/VTK,hendradarwin/VTK,sumedhasingla/VTK,sankhesh/VTK,demarle/VTK,jmerkow/VTK,berendkleinhaneveld/VTK,candy7393/VTK,aashish24/VTK-old,demarle/VTK,aashish24/VTK-old,hendradarwin/VTK,candy7393/VTK,keithroe/vtkoptix,sankhesh/VTK,gram526/VTK,mspark93/VTK,sankhesh/VTK,aashish24/VTK-old,biddisco/VTK,gram526/VTK,ashray/VTK-EVM,msmolens/VTK,cjh1/VTK,collects/VTK,mspark93/VTK,demarle/VTK,cjh1/VTK,msmolens/VTK,mspark93/VTK,mspark93/VTK,mspark93/VTK,SimVascular/VTK,gram526/VTK,daviddoria/PointGraphsPhase1,sankhesh/VTK,candy7393/VTK,aashish24/VTK-old,spthaolt/VTK,msmolens/VTK,keithroe/vtkoptix,keithroe/vtkoptix,collects/VTK,biddisco/VTK,candy7393/VTK,biddisco/VTK,jmerkow/VTK,johnkit/vtk-dev,jmerkow/VTK,candy7393/VTK,daviddoria/PointGraphsPhase1,hendradarwin/VTK,msmolens/VTK,biddisco/VTK,keithroe/vtkoptix,collects/VTK,sankhesh/VTK,spthaolt/VTK,aashish24/VTK-old,hendradarwin/VTK,johnkit/vtk-dev,daviddoria/PointGraphsPhase1,spthaolt/VTK,sumedhasingla/VTK,collects/VTK,hendradarwin/VTK,biddisco/VTK,collects/VTK,gram526/VTK,daviddoria/PointGraphsPhase1,berendkleinhaneveld/VTK,jmerkow/VTK,aashish24/VTK-old,johnkit/vtk-dev,mspark93/VTK,biddisco/VTK,sumedhasingla/VTK,keithroe/vtkoptix,berendkleinhaneveld/VTK,jmerkow/VTK,msmolens/VTK,SimVascular/VTK,cjh1/VTK,msmolens/VTK,keithroe/vtkoptix,cjh1/VTK,johnkit/vtk-dev,gram526/VTK,keithroe/vtkoptix,biddisco/VTK,berendkleinhaneveld/VTK,ashray/VTK-EVM,sumedhasingla/VTK,sankhesh/VTK,ashray/VTK-EVM,sumedhasingla/VTK,mspark93/VTK,demarle/VTK,johnkit/vtk-dev,candy7393/VTK,jmerkow/VTK,ashray/VTK-EVM,sankhesh/VTK,sumedhasingla/VTK,berendkleinhaneveld/VTK,gram526/VTK,ashray/VTK-EVM,SimVascular/VTK,ashray/VTK-EVM