The last time I ran ./contrib/download_prerequisites, I already had
previous symlinks set up from a previous run of the script, so `ln`
followed the existing symlinks and created the new ones in the
directories to which the symlinks pointed. This patch should fix that
by removing the old symlinks before creating new ones. (For some
reason the `-f` flag to `ln` that was already there wasn't enough for
me.) Tested by running the script and ensuring that the new isl
symlink pointed to the correct directory, and that there were no bad
symlinks in the old isl directory. Could someone commit this trivial
patch for me, or something like it? I don't have write access.

Thanks,
Eric Gallager