ExternalProject: Always run update step with non-empty UPDATE_COMMAND

The purpose of the `update` step is to run an update on each build
(subject to `UPDATE_DISCONNECTED`).  This is done for version-controlled
source directories.  We should do it for a custom `UPDATE_COMMAND` too.
In particular, when `UPDATE_DISCONNECTED` is used we expect the
`skip-update` step to exist.

Modules/ExternalProject.cmake

@@ -2030,6 +2030,9 @@ function(_ep_add_update_command name)
 
   if(cmd_set)
     set(work_dir ${source_dir})
+    if(NOT "x${cmd}" STREQUAL "x")
+      set(always 1)
+    endif()
   elseif(cvs_repository)
     if(NOT CVS_EXECUTABLE)
       message(FATAL_ERROR "error: could not find cvs for update of ${name}")

Tests/ExternalProjectUpdate/CMakeLists.txt

@@ -21,6 +21,18 @@ set_property(DIRECTORY PROPERTY EP_BASE ${base})
 set_property(DIRECTORY PROPERTY EP_STEP_TARGETS configure build test)
 set_property(DIRECTORY PROPERTY EP_INDEPENDENT_STEP_TARGETS update)
 
+ExternalProject_Add(TestUpdateCommand
+  SOURCE_DIR ${CMAKE_CURRENT_SOURCE_DIR}
+  UPDATE_COMMAND ${CMAKE_COMMAND} -E echo update
+  UPDATE_DISCONNECTED 1
+  CONFIGURE_COMMAND ""
+  BUILD_COMMAND ""
+  INSTALL_COMMAND ""
+  INDEPENDENT_STEP_TARGETS update
+  )
+add_custom_target(TestUpdateCommandDriver ALL)
+add_dependencies(TestUpdateCommandDriver TestUpdateCommand-update)
+
 set(do_git_tests 0)
 
 if(GIT_EXECUTABLE)