Wrapping/Tcl: Fix CMake Policy CMP0050 warnings

Avoid using the undocumented SOURCE option to add_custom_command.
Replace the ancient SOURCE == TARGET spelling with POST_BUILD.
While at it, use a $<TARGET_FILE> generator expression to refer
to the executable location of the vtk and pvtk targets.

Change-Id: I8fcd894ede6e430dc148dced98f3b404b9ae1101
IO/XML: Remove unnecessary <sstream> include

The VTK standard headers include <iosfwd> and make std::istringstream
available for use as an incomplete type.  That is all vtkXMLReader.h
needs.  Remove the extra <sstream> include.

In particular, by including it before the VTK headers it causes system
headers to be included with inconsistent Large File Support definitions
on AIX, leading to runtime shared library unresolved symbols.  Remove
the inclusion to fix this.

Change-Id: I78ec9efae24c8f721f079ab8946afcde413fe17f
Merge topic 'remove-unnecessary-include' into master

d8a74423 IO/XML: Remove unnecessary <sstream> include
Merge topic 'fix-CMP0050-warnings' into master

e2d31236 Wrapping/Tcl: Fix CMake Policy CMP0050 warnings