Добавление компилятора C в BusyBox

Не возможно. От официальный Busybox страница:

    Currently available applets include:

    [, [[, acpid, addgroup, adduser, adjtimex, ar, arp, arping, ash,
    awk, basename, beep, blkid, brctl, bunzip2, bzcat, bzip2, cal, cat,
    catv, chat, chattr, chgrp, chmod, chown, chpasswd, chpst, chroot,
    chrt, chvt, cksum, clear, cmp, comm, cp, cpio, crond, crontab,
    cryptpw, cut, date, dc, dd, deallocvt, delgroup, deluser, depmod,
    devmem, df, dhcprelay, diff, dirname, dmesg, dnsd, dnsdomainname,
    dos2unix, dpkg, du, dumpkmap, dumpleases, echo, ed, egrep, eject,
    env, envdir, envuidgid, expand, expr, fakeidentd, false, fbset,
    fbsplash, fdflush, fdformat, fdisk, fgrep, find, findfs, flash_lock,
    flash_unlock, fold, free, freeramdisk, fsck, fsck.minix, fsync,
    ftpd, ftpget, ftpput, fuser, getopt, getty, grep, gunzip, gzip, hd,
    hdparm, head, hexdump, hostid, hostname, httpd, hush, hwclock, id,
    ifconfig, ifdown, ifenslave, ifplugd, ifup, inetd, init, inotifyd,
    insmod, install, ionice, ip, ipaddr, ipcalc, ipcrm, ipcs, iplink,
    iproute, iprule, iptunnel, kbd_mode, kill, killall, killall5, klogd,
    last, length, less, linux32, linux64, linuxrc, ln, loadfont,
    loadkmap, logger, login, logname, logread, losetup, lpd, lpq, lpr,
    ls, lsattr, lsmod, lzmacat, lzop, lzopcat, makemime, man, md5sum,
    mdev, mesg, microcom, mkdir, mkdosfs, mkfifo, mkfs.minix, mkfs.vfat,
    mknod, mkpasswd, mkswap, mktemp, modprobe, more, mount, mountpoint,
    mt, mv, nameif, nc, netstat, nice, nmeter, nohup, nslookup, od,
    openvt, passwd, patch, pgrep, pidof, ping, ping6, pipe_progress,
    pivot_root, pkill, popmaildir, printenv, printf, ps, pscan, pwd,
    raidautorun, rdate, rdev, readlink, readprofile, realpath,
    reformime, renice, reset, resize, rm, rmdir, rmmod, route, rpm,
    rpm2cpio, rtcwake, run-parts, runlevel, runsv, runsvdir, rx, script,
    scriptreplay, sed, sendmail, seq, setarch, setconsole, setfont,
    setkeycodes, setlogcons, setsid, setuidgid, sh, sha1sum, sha256sum,
    sha512sum, showkey, slattach, sleep, softlimit, sort, split,
    start-stop-daemon, stat, strings, stty, su, sulogin, sum, sv,
    svlogd, swapoff, swapon, switch_root, sync, sysctl, syslogd, tac,
    tail, tar, taskset, tcpsvd, tee, telnet, telnetd, test, tftp, tftpd,
    time, timeout, top, touch, tr, traceroute, true, tty, ttysize,
    udhcpc, udhcpd, udpsvd, umount, uname, uncompress, unexpand, uniq,
    unix2dos, unlzma, unlzop, unzip, uptime, usleep, uudecode, uuencode,
    vconfig, vi, vlock, volname, watch, watchdog, wc, wget, which, who,
    whoami, xargs, yes, zcat, zcip