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
0cc1b550dd