Fix regression introduced by #9660

This commit restores the compiler's behaviour when called on a C file
and with no "-o" otption.

Changes

@@ -486,9 +486,9 @@ OCaml 4.13.0
 - #9407: added warning for missing mli interface file
   (Anukriti Kumar, review by Florian Angeletti)
 
-- #9960: extend ocamlc/ocamlopt's -o option to work when compiling C files
-  (Sébastien Hinderer, reported by Daniel Bünzli, review by Florian
-  Angeletti and Gabriel Scherer)
+- #9960, #10619: extend ocamlc/ocamlopt's -o option to work when
+  compiling C files (Sébastien Hinderer, reported by Daniel Bünzli,
+  review by Florian Angeletti and Gabriel Scherer)
 
 - #10095: simplify the syntax error messages produced by the compiler.
   In many cases, the compiler would produce an error message that looked

driver/compenv.ml

@@ -641,13 +641,18 @@ let process_action
   | ProcessCFile name ->
       readenv ppf (Before_compile name);
       Location.input_name := name;
-      let obj_name = match !output_name with
-        | None -> c_object_of_filename name
-        | Some n -> n
-      in
-      if Ccomp.compile_file ~output:obj_name name <> 0
-      then raise (Exit_with_status 2);
-      ccobjs := obj_name :: !ccobjs
+      (match !output_name with
+      | None ->
+        begin
+          if Ccomp.compile_file name <> 0 then raise (Exit_with_status 2);
+          ccobjs := c_object_of_filename name :: !ccobjs
+        end
+      | Some obj_name ->
+        begin
+          if Ccomp.compile_file ~output:obj_name name <> 0
+          then raise (Exit_with_status 2);
+          ccobjs := obj_name :: !ccobjs
+        end)
   | ProcessObjects names ->
       ccobjs := names @ !ccobjs
   | ProcessDLLs names ->