ExternalProject: support extracting the configure command

Previously, the configure command generated by ExternalProject was not
accessible prior to actually adding the targets. This makes the CMake
configure command accessible with just a call to _ep_parse_arguments.

Future work will leverage this to support custom environment settings on
a per-project basis.

Modules/ExternalProject.cmake

@@ -1180,6 +1180,7 @@ function(_ep_write_initial_cache target_name script_filename script_initial_cach
   # Write out values into an initial cache, that will be passed to CMake with -C
   # Replace location tags.
   _ep_replace_location_tags(${target_name} script_initial_cache)
+  _ep_replace_location_tags(${target_name} script_filename)
   # Write out the initial cache file to the location specified.
   file(GENERATE OUTPUT "${script_filename}" CONTENT "${script_initial_cache}")
 endfunction()
@@ -2181,24 +2182,7 @@ function(_ep_add_patch_command name)
 endfunction()
 
 
-# TODO: Make sure external projects use the proper compiler
-function(_ep_add_configure_command name)
-  ExternalProject_Get_Property(${name} source_dir source_subdir binary_dir tmp_dir)
-
-  # Depend on other external projects (file-level).
-  set(file_deps)
-  get_property(deps TARGET ${name} PROPERTY _EP_DEPENDS)
-  foreach(dep IN LISTS deps)
-    get_property(dep_type TARGET ${dep} PROPERTY TYPE)
-    if(dep_type STREQUAL "UTILITY")
-      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()
-    endif()
-  endforeach()
-
+function(_ep_extract_configure_command var name)
   get_property(cmd_set TARGET ${name} PROPERTY _EP_CONFIGURE_COMMAND SET)
   if(cmd_set)
     get_property(cmd TARGET ${name} PROPERTY _EP_CONFIGURE_COMMAND)
@@ -2219,7 +2203,7 @@ function(_ep_add_configure_command name)
     get_property(cmake_cache_default_args TARGET ${name} PROPERTY _EP_CMAKE_CACHE_DEFAULT_ARGS)
 
     if(cmake_cache_args OR cmake_cache_default_args)
-      set(_ep_cache_args_script "${tmp_dir}/${name}-cache-$<CONFIG>.cmake")
+      set(_ep_cache_args_script "<TMP_DIR>/${name}-cache-$<CONFIG>.cmake")
       if(cmake_cache_args)
         _ep_command_line_to_initial_cache(script_initial_cache_force "${cmake_cache_args}" 1)
       endif()
@@ -2261,9 +2245,32 @@ function(_ep_add_configure_command name)
       endif()
     endif()
 
-    list(APPEND cmd "${source_dir}${source_subdir}")
+    list(APPEND cmd "<SOURCE_DIR><SOURCE_SUBDIR>")
   endif()
 
+  set("${var}" "${cmd}" PARENT_SCOPE)
+endfunction()
+
+# TODO: Make sure external projects use the proper compiler
+function(_ep_add_configure_command name)
+  ExternalProject_Get_Property(${name} binary_dir tmp_dir)
+
+  # Depend on other external projects (file-level).
+  set(file_deps)
+  get_property(deps TARGET ${name} PROPERTY _EP_DEPENDS)
+  foreach(dep IN LISTS deps)
+    get_property(dep_type TARGET ${dep} PROPERTY TYPE)
+    if(dep_type STREQUAL "UTILITY")
+      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()
+    endif()
+  endforeach()
+
+  _ep_extract_configure_command(cmd ${name})
+
   # If anything about the configure command changes, (command itself, cmake
   # used, cmake args or cmake generator) then re-run the configure step.
   # Fixes issue https://gitlab.kitware.com/cmake/cmake/issues/10258