ExternalProject: Initialize only selected git submodules (#15590)

Honor the GIT_SUBMODULES option added by commit v3.0.0-rc1~92^2
(ExternalProject: Add option GIT_SUBMODULES, 2014-01-09) during the
'git submodule init' step to avoid initalizing all modules since we
are only going to update the specified subset anyway.  This will
be useful for project repositories that have many submodules
(e.g. https://github.com/boostorg/boost).

Reviewed-by: Daniele E. Domenichelli <[email protected]>

Modules/ExternalProject.cmake

@@ -529,7 +529,7 @@ if(error_code)
 endif()
 
 execute_process(
-  COMMAND \"${git_EXECUTABLE}\" submodule init
+  COMMAND \"${git_EXECUTABLE}\" submodule init ${git_submodules}
   WORKING_DIRECTORY \"${work_dir}/${src_name}\"
   RESULT_VARIABLE error_code
   )