busybox

- fix bug where we linked again -lm even though it is not needed.
  For BBOX_LIB_LIST="crypt m" trylink ... with just applet true, we pulled in
  libm because in this case we tried to use invalid flags (plain "-l" without
  a lib) which of course failed, thus the script thought that -lm was needed.

  The fix is not to pass "-l" without a lib if we are about to check if any
  or the last remaining lib is really needed.