[Cmake] SOURCE_FILES_REMOVE

[John Biddiscombe]

I've just added a new command SOURCE_FILES_REMOVE. It performs the opposite 
of SOURCE_FILES(...) and simply removes the sources from the underlying map 
container.

example of usage...

SOURCE_FILES_REMOVE(SRCS vtkVRMLImporter)

combined with Bill's earlier INCLUDE (... OPTIONAL) command it allows one 
to customize libraries by removing certain files from the build - without 
constant editing of the makelists files (resolving conflicts from CVS).

JB