CLISP - an ANSI Common Lisp

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I removed --install altogether; this closes the issue

I agree that that detail of the installation phase is better left up to the
user.

The original configure script entered the su case on my MacOSX just
because the prefix directory didn't exist yet. (Without the call to su in
configure, I would have used sudo to create the directory and change
ownership, and I would have installed it under my user login).

yeah, sounds like the best solution is to remove the "install" command
altogether.
I can hardly imagine an admin who would do that (Pascal, how did you
stumble upon this?)
objections?

'root' does not need to exist or have a password. On the MacOS X 10.5 box I
have access to, no one has the password of user "root". Only the user
"admin" has a password and administrative privileges.

'sudo' can also not necessarily be used, because it fails for users
outside the "wheel" group.

Also, some users might not want that scripts ask for administrative
privileges without explaining for what tasks they need them.

In summary, I would move all code that requires administrative privileges
to a separate script, and tell the user to execute that script, in a way
appropriate to his environment.

At least su root -c "make install" works both on Darwin and Linux.
(su is not a POSIX standard command, so we can only pray for some
commonality here).

I think "su root -c foo" would work on other platforms too.

Notice the syntax of su on Darwin. Anything following the login name is
passed to the shell. The words -c make install given to su on Darwin will
be passed to bash, which will thus execute bash -c make install

However, this is still wrong, since this gives only make to -c, and
install is used to fill argv by bash.
We could group in a single word the arguments to my_su:

my_su "make install"

(for a generic function we would have to quote-shell-argument the words in
my_su to be able to concatenate them in a single string to give with -c).

your version also uses "-c".