feat(configh): generate a guarded C header from probe results

Emits the project name and the HAVE_ and SIZEOF_ defines from what the probes
actually found, behind an include guard derived from the output path.

lib/configh.ml

@@ -0,0 +1,33 @@
+open Types
+
+let guard p =
+  let base = Ident.cident (Filename.basename (Option.value p.config_header ~default:"config.h")) in
+  Printf.sprintf "MEOWC_%s_%s" (Ident.cident p.pname) base
+
+let render p =
+  let b = Buffer.create 1024 in
+  let g = guard p in
+  Buffer.add_string b "/* Generated by meowc. Edits will be overwritten. */\n";
+  Buffer.add_string b (Printf.sprintf "#ifndef %s\n#define %s\n\n" g g);
+  Buffer.add_string b (Printf.sprintf "#define %s_NAME %S\n\n" (Ident.cident p.pname) p.pname);
+  let seen = Hashtbl.create 32 in
+  List.iter
+    (fun d ->
+      if not (Hashtbl.mem seen d.dname) then begin
+        Hashtbl.add seen d.dname ();
+        match d.dvalue with
+        | Some v -> Buffer.add_string b (Printf.sprintf "#define %s %s\n" d.dname v)
+        | None -> Buffer.add_string b (Printf.sprintf "/* #undef %s */\n" d.dname)
+      end)
+    p.defines;
+  Buffer.add_string b (Printf.sprintf "\n#endif /* %s */\n" g);
+  Buffer.contents b
+
+let emit p =
+  match p.config_header with
+  | None -> None
+  | Some path ->
+      let text = render p in
+      let changed = match Fs.read path with exception Sys_error _ -> true | old -> old <> text in
+      if changed then Fs.write path text;
+      Some (path, changed)