cmake: move VTK_FORBID_DOWNLOADS to its own module

CMake/vtkDownload.cmake

+# Forbid downloading resources from the network during a build. This helps
+# when building on systems without network connectivity to determine which
+# resources much be obtained manually and made available to the build.
+option(VTK_FORBID_DOWNLOADS "Do not download source code or data from the network" OFF)
+mark_as_advanced(VTK_FORBID_DOWNLOADS)
+macro(vtk_download_attempt_check _name)
+  if(VTK_FORBID_DOWNLOADS)
+    message(SEND_ERROR "Attempted to download ${_name} when VTK_FORBID_DOWNLOADS is ON")
+  endif()
+endmacro()

CMakeLists.txt

@@ -14,6 +14,7 @@ include(vtkInitializeBuildType)
 include(vtkVersion)
 set(VTK_VERSION
     "${VTK_MAJOR_VERSION}.${VTK_MINOR_VERSION}.${VTK_BUILD_VERSION}")
+include(vtkDownload)
 
 # Objective-C++ compile flags.
 # CMake has no equivalent of CMAKE_CXX_FLAGS for Objective-C++ (bug #4756)
@@ -29,18 +30,6 @@ endif()
 include(vtkModuleMacros)
 include(vtkExternalData)
 
-#-----------------------------------------------------------------------------
-# Forbid downloading resources from the network during a build. This helps
-# when building on systems without network connectivity to determine which
-# resources much be obtained manually and made available to the build.
-option(VTK_FORBID_DOWNLOADS "Do not download source code or data from the network" OFF)
-mark_as_advanced(VTK_FORBID_DOWNLOADS)
-macro(vtk_download_attempt_check _name)
-  if(VTK_FORBID_DOWNLOADS)
-    message(SEND_ERROR "Attempted to download ${_name} when VTK_FORBID_DOWNLOADS is ON")
-  endif()
-endmacro()
-
 # Test input data staging directory.
 file(RELATIVE_PATH vtk_reldir "${CMAKE_SOURCE_DIR}" "${CMAKE_CURRENT_SOURCE_DIR}")
 set(VTK_TEST_DATA_DIR "${ExternalData_BINARY_ROOT}/${vtk_reldir}/Testing")