gnu: vtk: Remove some vendored libraries.

* gnu/packages/image-processing.scm (vtk)[source]: Add snippet to remove
some vendored sources.

gnu/packages/image-processing.scm

@@ -7,7 +7,7 @@
 ;;; Copyright © 2018, 2019, 2020 Tobias Geerinckx-Rice <me@tobias.gr>
 ;;; Copyright © 2018 Björn Höfling <bjoern.hoefling@bjoernhoefling.de>
 ;;; Copyright © 2018 Lprndn <guix@lprndn.info>
-;;; Copyright © 2019 Efraim Flashner <efraim@flashner.co.il>
+;;; Copyright © 2019, 2021 Efraim Flashner <efraim@flashner.co.il>
 ;;; Copyright © 2020 Vincent Legoll <vincent.legoll@gmail.com>
 ;;; Copyright © 2020 Vinicius Monego <monego@posteo.net>
 ;;; Copyright © 2020 Pierre Neidhardt <mail@ambrevar.xyz>
@@ -252,7 +252,17 @@ many popular formats.")
                                   "/VTK-" version ".tar.gz"))
               (sha256
                (base32
-                "1fspgp8k0myr6p2a6wkc21ldcswb4bvmb484m12mxgk1a9vxrhrl"))))
+                "1fspgp8k0myr6p2a6wkc21ldcswb4bvmb484m12mxgk1a9vxrhrl"))
+              (modules '((guix build utils)))
+              (snippet
+               '(begin
+                  (for-each
+                    (lambda (dir)
+                      (delete-file-recursively
+                        (string-append "ThirdParty/" dir "/vtk" dir)))
+                    '("expat" "freetype" "hdf5" "jpeg" "jsoncpp" "libxml2"
+                      "png" "tiff" "zlib"))
+                  #t))))
     (build-system cmake-build-system)
     (arguments
      '(#:build-type "Release"           ;Build without '-g' to save space.