gnu: vtk-7: Fix build.

* gnu/packages/patches/vtk-7-gcc-11-compat.patch: New file.
* gnu/local.mk (dist_patch_DATA): Register it.
* gnu/packages/image-processing.scm (vtk-7)[source]: Add patch for gcc-11.

gnu/local.mk

@@ -2085,6 +2085,7 @@ dist_patch_DATA =						\
   %D%/packages/patches/vte-CVE-2012-2738-pt1.patch			\
   %D%/packages/patches/vte-CVE-2012-2738-pt2.patch			\
   %D%/packages/patches/vtk-7-gcc-10-compat.patch		\
+  %D%/packages/patches/vtk-7-gcc-11-compat.patch		\
   %D%/packages/patches/vtk-7-hdf5-compat.patch			\
   %D%/packages/patches/vtk-7-python-compat.patch		\
   %D%/packages/patches/wacomtablet-add-missing-includes.patch	\

gnu/packages/image-processing.scm

@@ -473,7 +473,8 @@ integrates with various databases on GUI toolkits such as Qt and Tk.")
                 "0nm7xwwj7rnsxjdv2ssviys8nhci4n9iiiqm2y14s520hl2dsp1d"))
               (patches (search-patches "vtk-7-python-compat.patch"
                                        "vtk-7-hdf5-compat.patch"
-                                       "vtk-7-gcc-10-compat.patch"))))
+                                       "vtk-7-gcc-10-compat.patch"
+                                       "vtk-7-gcc-11-compat.patch"))))
     (arguments
      (substitute-keyword-arguments (package-arguments vtk)
        ((#:configure-flags flags)

gnu/packages/patches/vtk-7-gcc-11-compat.patch

@@ -0,0 +1,11 @@
+Description: GCC 11 now enforces that comparison objects be invocable as const.
+--- a/Rendering/Label/vtkLabelHierarchy.cxx
++++ b/Rendering/Label/vtkLabelHierarchy.cxx
+@@ -525,1 +525,1 @@
+-                    const vtkHierarchyNode & b)
++                    const vtkHierarchyNode & b) const
+--- a/Rendering/Label/vtkLabelHierarchyPrivate.h
++++ b/Rendering/Label/vtkLabelHierarchyPrivate.h
+@@ -66,1 +66,1 @@
+-    bool operator () ( const vtkIdType& a, const vtkIdType& b )
++    bool operator () ( const vtkIdType& a, const vtkIdType& b ) const