set GDCM_VTK_DIR to VTK_DIR which is exported into *.cmake files
introduce new variable GDCM_SKIP_USE_VTK

When set to true, it skips all parts which makes a cmake dependency to VTK
(when GDCM is built with GDCM_USE_VTK turned ON). Note by default this variable
is not set (which is equivalent to set to false).

Here is an example of usage:

  set( GDCM_SKIP_USE_VTK ON )

  find_package( GDCM REQUIRED )
  include( ${GDCM_USE_FILE} )