[Udpcast] compile udpcast failed

I list symbol from udpcdialog/lib.a 1) cd busybox-1.13.2/udpcdialog 2) nm lib.a | grep "_IO_cleanup" get result: U _IO_cleanup 00000000 T __bring_in_IO_cleanup 00000000 d __elf_set___libc_atexit_element__IO_cleanup__ _IO_cleanup not defined in busybox-1.13.2/udpcdialog/lib.a why ???