ExternalProject: Allow DEPENDS on normal targets (#13849)

The ExternalProject_Add DEPENDS option adds two types of dependencies.
It adds a target-level build order dependency between the external
project target and the named targets.  It also adds a file-level
dependency on the "done" stamp file of the named external project
targets.  Targets not created by ExternalProject_Add have no such stamp
file and no _EP_STAMP_DIR property.  Prior to commit d14c0243 (Refactor
repeated code into function, 2012-04-26) we unconditionally accepted an
empty stamp dir and generated a dependency on a non-existent file.
After that commit we generate an error that no stamp dir is set.

Skip the file-level dependency when the named dependency is not an
external project target in order to allow this use case.  Teach the
ExternalProject test to cover the case.

Modules/ExternalProject.cmake

@@ -1558,8 +1558,11 @@ function(_ep_add_configure_command name)
   set(file_deps)
   get_property(deps TARGET ${name} PROPERTY _EP_DEPENDS)
   foreach(dep IN LISTS deps)
-    _ep_get_step_stampfile(${dep} "done" done_stamp_file)
-    list(APPEND file_deps ${done_stamp_file})
+    get_property(is_ep TARGET ${dep} PROPERTY _EP_IS_EXTERNAL_PROJECT)
+    if(is_ep)
+      _ep_get_step_stampfile(${dep} "done" done_stamp_file)
+      list(APPEND file_deps ${done_stamp_file})
+    endif()
   endforeach()
 
   get_property(cmd_set TARGET ${name} PROPERTY _EP_CONFIGURE_COMMAND SET)

Tests/ExternalProject/CMakeLists.txt

@@ -50,6 +50,8 @@ if(NOT DEFINED can_build_tutorial_step5)
   endif()
 endif()
 
+add_custom_target(NonExternalProjectTarget
+  COMMAND ${CMAKE_COMMAND} -E echo NonExternalProjectTarget)
 
 # Empty projects that test all the known ExternalProject_Add argument key words:
 #
@@ -94,7 +96,7 @@ ExternalProject_Add(${proj}
   CVS_REPOSITORY ""
   CVS_MODULE ""
   CVS_TAG ""
-  DEPENDS "MinimalNoOpProject"
+  DEPENDS "MinimalNoOpProject" NonExternalProjectTarget
   DOWNLOAD_COMMAND ""
   INSTALL_COMMAND ""
   PATCH_COMMAND ""