If the name of the link is the same except for case, use install -S to
avoid removing the destination file (which will be the source on a
case-insensitive file system) before creating the link.
I've also modified install.sh to skip the ln call if the file names are
the same in lower case and test -ef (same file) returns true.
Without this change installworld fails on MacOS in libcrypto (due to hmac.3
-> HMAC.3) and sendmail (mail -> Mail)