ExternalProject,FetchContent: Avoid CMAKE_DISABLE_SOURCE_CHANGES error

The file(MAKE_DIRECTORY) implementation checks whether a path is
allowed to be written to before it checks if it already exists. For the
scenario where a SOURCE_DIR is an existing directory within the main
project's source directory, this triggers a fatal error if
CMAKE_DISABLE_SOURCE_CHANGES is set to true for ExternalProject,
and some FetchContent scenarios. Therefore, add an explicit check for
existence first to avoid making such error-triggering calls.

Fixes: #21872

Modules/ExternalProject/mkdirs.cmake.in

@@ -3,8 +3,13 @@
 
 cmake_minimum_required(VERSION 3.5)
 
+# If CMAKE_DISABLE_SOURCE_CHANGES is set to true and the source directory is an
+# existing directory in our source tree, calling file(MAKE_DIRECTORY) on it
+# would cause a fatal error, even though it would be a no-op.
+if(NOT EXISTS "@source_dir@")
+  file(MAKE_DIRECTORY "@source_dir@")
+endif()
 file(MAKE_DIRECTORY
-  "@source_dir@"
   "@binary_dir@"
   "@install_dir@"
   "@tmp_dir@"

Modules/FetchContent.cmake

@@ -1657,8 +1657,13 @@ function(__FetchContent_populateDirect)
   set(_EP_TMP_DIR   "${FETCHCONTENT_BASE_DIR}/${contentNameLower}-tmp")
   set(_EP_DOWNLOAD_DIR "${_EP_TMP_DIR}")
 
+  # If CMAKE_DISABLE_SOURCE_CHANGES is set to true and _EP_SOURCE_DIR is an
+  # existing directory in our source tree, calling file(MAKE_DIRECTORY) on it
+  # would cause a fatal error, even though it would be a no-op.
+  if(NOT EXISTS "${_EP_SOURCE_DIR}")
+    file(MAKE_DIRECTORY "${_EP_SOURCE_DIR}")
+  endif()
   file(MAKE_DIRECTORY
-    "${_EP_SOURCE_DIR}"
     "${_EP_BINARY_DIR}"
     "${_EP_STAMP_DIR}"
     "${_EP_TMP_DIR}"