Search for Ghostscript only when needed

Move the search for the Ghostscript executable added by commit 870dd114
(Add new GL2PS testing system using ghostscript, 2012-10-01) into a
dedicated vtkGhostscript module.  Include it from CMakeLists.txt that
need VTK_GHOSTSCRIPT_EXECUTABLE.  This avoids keeping module-specific
code in the top-level CMakeLists.txt file and enables the corresponding
tests in the Testing/External build.

Change-Id: I410737f16c653aaaa60799ce1471cb1798707079
Merge topic 'cleanup-Ghostscript-search' into master

fb82c67a Search for Ghostscript only when needed