Fix GL2PS building with OSMesa.
Fix GL2PS building with OSMesa.

The GL2PS module needed to be modified to use the OSMesa library
and headers when needed.

Change-Id: I3ee5d7c60aab1ff2e875cc0607dccbd406095a61

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