vtk_common: Store ExternalData persistently

Set ExternalData_OBJECT_STORES to a location outside the source and
build trees.  The location will persist across multiple runs and be
shared by all builds in the same dashboard root directory.  Since the
content of the directory is content-addressed and the objects are
stateless this should not affect test results.  It will however avoid
duplicate downloads reduce cases of problems due to network failure.

vtk_common.cmake

@@ -29,6 +29,7 @@
 #   dashboard_root_name       = Change name of "My Tests" directory
 #   dashboard_source_name     = Name of source directory (VTK)
 #   dashboard_binary_name     = Name of binary directory (VTK-build)
+#   dashboard_store_name      = Name of ExternalData store (ExternalData)
 #   dashboard_data_name       = Name of data directory (VTKData)
 #   dashboard_large_data_name = Name of data directory (VTKLargeData)
 #   dashboard_cache           = Initial CMakeCache.txt file content
@@ -60,7 +61,7 @@
 #   set(ENV{LD_LIBRARY_PATH} /path/to/vendor/lib) # (if necessary)
 
 #=============================================================================
-# Copyright 2010-2012 Kitware, Inc.
+# Copyright 2010-2013 Kitware, Inc.
 # All rights reserved.
 #
 # Redistribution and use in source and binary forms, with or without
@@ -199,6 +200,15 @@ if(NOT DEFINED CTEST_BINARY_DIRECTORY)
   endif()
 endif()
 
+# Select a data store.
+if(NOT DEFINED ExternalData_OBJECT_STORES)
+  if(DEFINED dashboard_store_name)
+    set(ExternalData_OBJECT_STORES ${CTEST_DASHBOARD_ROOT}/${dashboard_store_name})
+  else()
+    set(ExternalData_OBJECT_STORES ${CTEST_DASHBOARD_ROOT}/ExternalData)
+  endif()
+endif()
+
 # Select a data directory name.
 
 # don't do this until VTKData is actually a submodule.
@@ -360,6 +370,7 @@ foreach(v
     CTEST_CHECKOUT_COMMAND
     CTEST_SCRIPT_DIRECTORY
     CTEST_USE_LAUNCHERS
+    ExternalData_OBJECT_STORES
     VTK_DATA_ROOT
     VTK_LARGE_DATA_ROOT
     )
@@ -385,6 +396,7 @@ SITE:STRING=${CTEST_SITE}
 BUILDNAME:STRING=${CTEST_BUILD_NAME}
 CTEST_USE_LAUNCHERS:BOOL=${CTEST_USE_LAUNCHERS}
 DART_TESTING_TIMEOUT:STRING=${CTEST_TEST_TIMEOUT}
+ExternalData_OBJECT_STORES:STRING=${ExternalData_OBJECT_STORES}
 VTK_DATA_ROOT:PATH=${VTK_DATA_ROOT}
 VTK_LARGE_DATA_ROOT:PATH=${VTK_LARGE_DATA_ROOT}
 ${cache_build_type}