ExternalProject: Fix regression in stamp creation for Xcode+iOS

The change to `_ep_add_mkdir_command` in commit 5fbac2bb24
(ExternalProject: Move inline scripts to separate files, 2022-01-22,
v3.23.0-rc1~101^2) did not account for the possibility that
`CMAKE_CFG_INTDIR` is `$(CONFIGURATION)$(EFFECTIVE_PLATFORM_NAME)`
instead of just the configuration name.  Pass the value into the helper
script on the command line so that the native buildsystem placeholders
are evaluated.

Fixes: #23645

Modules/ExternalProject.cmake

@@ -1651,6 +1651,7 @@ function(_ep_set_directories name)
     ${script_filename}
     @ONLY
   )
+  unset(cfgdir) # do not leak into mkdirs.cmake script
   include(${script_filename})
 endfunction()
 
@@ -2419,11 +2420,12 @@ endfunction()
 function(_ep_add_mkdir_command name)
   ExternalProject_Get_Property(${name} tmp_dir)
   set(script_filename "${tmp_dir}/${name}-mkdirs.cmake")
+  _ep_get_configuration_subdir_suffix(cfgdir)
 
   ExternalProject_Add_Step(${name} mkdir
     INDEPENDENT TRUE
     COMMENT "Creating directories for '${name}'"
-    COMMAND ${CMAKE_COMMAND} -P ${script_filename}
+    COMMAND ${CMAKE_COMMAND} -Dcfgdir=${cfgdir} -P ${script_filename}
   )
 endfunction()
 

Modules/ExternalProject/mkdirs.cmake.in

@@ -17,3 +17,6 @@ set(configSubDirs @CMAKE_CONFIGURATION_TYPES@)
 foreach(subDir IN LISTS configSubDirs)
     file(MAKE_DIRECTORY "@stamp_dir@/${subDir}")
 endforeach()
+if(cfgdir)
+  file(MAKE_DIRECTORY "@stamp_dir@${cfgdir}") # cfgdir has leading slash
+endif()