build: older git versions don't have '-C' flag

CMakeLists.txt

@@ -176,8 +176,8 @@ if(NOT MANUAL_SUBMODULES)
   find_package(Git)
   if(GIT_FOUND)
     function (check_submodule relative_path)
-      execute_process(COMMAND git -C ${relative_path} rev-parse "HEAD"  WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR} OUTPUT_VARIABLE localHead)
-      execute_process(COMMAND git rev-parse "HEAD:${relative_path}"     WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR} OUTPUT_VARIABLE checkedHead)
+      execute_process(COMMAND git rev-parse "HEAD" WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}/${relative_path} OUTPUT_VARIABLE localHead)
+      execute_process(COMMAND git rev-parse "HEAD:${relative_path}" WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR} OUTPUT_VARIABLE checkedHead)
       string(COMPARE EQUAL "${localHead}" "${checkedHead}" upToDate)
       if (upToDate)
         message(STATUS "Submodule '${relative_path}' is up-to-date")