[default] Make books work on 0.4.15 again
[default] Make books work on 0.4.15 again

 - Check for the presence of new API before using it
 - For #536

LeMagnesium/minetest-minetestforfun-server,MinetestForFun/minetest-minetestforfun-server,MinetestForFun/server-minetestforfun,MinetestForFun/minetest-minetestforfun-server,MinetestForFun/server-minetestforfun,MinetestForFun/minetest-minetestforfun-server,MinetestForFun/server-minetestforfun,Ombridride/minetest-minetestforfun-server,Ombridride/minetest-minetestforfun-server,LeMagnesium/minetest-minetestforfun-server,LeMagnesium/minetest-minetestforfun-server,Ombridride/minetest-minetestforfun-server