Fix build for vtk6.1.0

var/spack/repos/builtin/packages/vtk/package.py

@@ -39,6 +39,10 @@ def install(self, spec, prefix):
             if spec['qt'].satisfies('@5'):
                 cmake_args.append("-DVTK_QT_VERSION:STRING=5")
 
+            if spec.satisfies("@6.1.0"):
+                cmake_args.append("-DCMAKE_C_FLAGS=-DGLX_GLXEXT_LEGACY")
+                cmake_args.append("-DCMAKE_CXX_FLAGS=-DGLX_GLXEXT_LEGACY")
+
             cmake(*cmake_args)
             make()
             make("install")