ENH: Rewrite FindR.cmake to be more robust
ENH: Rewrite FindR.cmake to be more robust

Using a non-installed instance of R should now work properly. Libraries for
R are now searched for in the priority:
  1. Within a user-built instance of R at VTK_R_HOME
  2. Within an installed instance of R
  3. Within external system libraries

R_LIBRARY_BLAS and R_LIBRARY_LAPACK will now be found in external
system libraries, if R does not provide them directly.

If R cannot be located, a slightly more specific error message is printed.

Change-Id: I8094d5ba3b3acebe880764b6108433917dd18376

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