fix -mpopcnt compilation failure on macOS/aarch64 10.4

The problem arises from this bug in clang
llvm/llvm-project#116278
which causes the following:

```
$ cc t.c -mpopcnt
clang: error: unsupported option '-mpopcnt' for target 'arm64-apple-darwin24.4.0'
$ cc t.c -mpopcnt -lpthread
```

The src/discover uses dune configurator, which always appends
the C link flags from ocamlopt to the command line, and so the
configurator erronously thinks that -mpopcnt will work.

Since the dune configurator can't easily be overridden, this
PR works around this specific link order issue by hand-calling
the cc compiler from the discover binary.

fixes #164 and #168

Author: avsm

src/discover/discover.ml

@@ -14,7 +14,11 @@ let () =
   main
     ~name:"discover"
     ~args:[ "-o", Set_string output, "FILENAME output file" ]
-    (fun c ->
-       let has_popcnt = c_test c ~c_flags:[ "-mpopcnt" ] program in
+    (fun _c ->
+       let f,oc = Filename.open_temp_file "baseconf" ".c" in
+       let has_popcnt =
+         Fun.protect ~finally:(fun () -> close_out oc; Sys.remove f)
+           (fun () -> output_string oc program; flush oc;
+             Sys.command (Printf.sprintf "cc %s -mpopcnt -o /dev/null >/dev/null 2>&1" f) = 0) in
        Flags.write_sexp !output (if has_popcnt then [ "-mpopcnt" ] else []))
 ;;