Detect VTK 6.0
Use AC_LINK_IFELSE instead of AC_HAVE_LIBRARY for VTK-6.0
Ran bootstrap.
Fix comparison operator.

I think you need double parens if you want to use '<' or '>' in
an m4 file.  On my system it was writing a empty file called "5".
Avoid 'No such file or directory' warning when configuring VTK-5.
The VTK SetInput() member apparently got renamed to SetInputData()?
We also use vtkXMLPUnstructuredGridWriter.

Therefore we must also link against libvtkIOXML.
VTK API change was already in 6.0
Ignore warnings from VTK-6 header files.
Merge pull request #265 from libMesh/vtk6_compat

VTK 6 compatibility