optimize performance of the build in a fresh clone

don't fetch the complete history of all submodules, it's rarely needed

cmake/submodules.cmake

@@ -19,11 +19,11 @@ IF(GIT_EXECUTABLE AND EXISTS "${CMAKE_SOURCE_DIR}/.git")
     SET(update_result 0)
   ELSEIF (cmake_update_submodules MATCHES force)
     MESSAGE(STATUS "Updating submodules (forced)")
-    EXECUTE_PROCESS(COMMAND "${GIT_EXECUTABLE}" submodule update --init --force
+    EXECUTE_PROCESS(COMMAND "${GIT_EXECUTABLE}" submodule update --init --force --depth=1
                     WORKING_DIRECTORY "${CMAKE_SOURCE_DIR}"
                     RESULT_VARIABLE update_result)
   ELSEIF (cmake_update_submodules MATCHES yes)
-    EXECUTE_PROCESS(COMMAND "${GIT_EXECUTABLE}" submodule update --init
+    EXECUTE_PROCESS(COMMAND "${GIT_EXECUTABLE}" submodule update --init --depth=1
                     WORKING_DIRECTORY "${CMAKE_SOURCE_DIR}"
                     RESULT_VARIABLE update_result)
   ELSE()