ExternalProject: Always use $<CONFIG> for source files

Modules/ExternalProject.cmake

@@ -2091,13 +2091,7 @@ function(_ep_get_configuration_subdir_genex suffix_var)
   set(suffix "")
   get_property(_isMultiConfig GLOBAL PROPERTY GENERATOR_IS_MULTI_CONFIG)
   if(_isMultiConfig)
-    if(CMAKE_GENERATOR STREQUAL "Xcode")
-      # The Xcode generator does not support per-config sources,
-      # so use the underlying build system's placeholder instead.
-      set(suffix "/${CMAKE_CFG_INTDIR}")
-    else()
-      set(suffix "/$<CONFIG>")
-    endif()
+    set(suffix "/$<CONFIG>")
   endif()
   set(${suffix_var} "${suffix}" PARENT_SCOPE)
 endfunction()