CONFIG: prefer use of ParaView_MESA_DIR in runTimePostProcessing

- when using VTK from ParaView sources it can better to tag them as
  such, but simultaneously not mask the ParaView with hardware
  rendering.

  The additional ParaView_MESA_DIR variable allows this.
  The balance of library and path setup is unaffected by this.

DOC: update doc/BuildIssues

src/functionObjects/graphics/runTimePostProcessing/Allwmake

@@ -8,6 +8,12 @@ echo "${PWD##*/} : $PWD"
 
 unset depend runTimePostProDebug
 
+# If available, prefer use of ParaView MESA
+if [ -d "$ParaView_MESA_DIR" ]
+then
+    export ParaView_DIR="$ParaView_MESA_DIR"
+fi
+
 if [ -d "$VTK_DIR" ]
 then
     depend="VTK_DIR=$VTK_DIR"