Andrew, thank you to point me out luaL_requiref and to suggest me to see the source code.
This it luaL_openlibs source code, very instructive:
for (lib = loadedlibs; lib->func; lib++) {
luaL_requiref(L, lib->name, lib->func, 1);
lua_pop(L, 1); /* remove lib */
So the solution for LUA 5.3 is
luaL_requiref(L, "_G", luaopen_base, 1);
luaL_requiref(L, LUA_STRLIBNAME, luaopen_string, 1);
And for LUA 5.4 :
luaL_requiref(L,
LUA_GNAME, luaopen_base, 1);
luaL_requiref(L, LUA_STRLIBNAME, luaopen_string, 1);
M