What's the correct way to invoke `clang`?

In debugging #714 I attempted to set the compiler used by ecl to clang via these instructions: https://ecl.common-lisp.dev/static/manual/System-building.html#C-compiler-configuration

I set:

(setf c::*cc* "clang")

but unfortunately this doesn't seem to work as-is. Upon loading any package (after deleting all caches!), I'm told:

/tmp/eclinitULdAwx.o: file not recognized: file format not recognized

Something else that's curious is the command it seems to attempt invoking:

Error code 1 when executing
(EXT:RUN-PROGRAM "gcc" ("-o" ....

Do you know why it would be attempting gcc here again?

System Information

     VERSION "21.2.1"
      VCS-ID "UNKNOWN"
          OS "Linux"
  OS-VERSION "6.4.12-arch1-1"
MACHINE-TYPE "x86_64"
    FEATURES (:QUICKLISP :ASDF-PACKAGE-SYSTEM :ASDF3.1 :ASDF3 :ASDF2 :ASDF :OS-UNIX :NON-BASE-CHARS-EXIST-P :ASDF-UNICODE :WALKER :CDR-1 :CDR-5 :LINUX :FORMATTER :CDR-7 :ECL-WEAK-HASH ...)