CreateProcess needs CREATE_UNICODE_ENVIRONMENT

> If this flag is set, the environment block pointed to by
> lpEnvironment uses Unicode characters. Otherwise, the environment
> block uses ANSI characters.

[doc][]

This was forgotten from PR ocsigen#843 that switched to Unicode strings on
Windows (including the environment).

[doc]: https://docs.microsoft.com/en-us/windows/win32/procthread/process-creation-flags#CREATE_UNICODE_ENVIRONMENT

CHANGES

@@ -41,7 +41,7 @@
 
   * Code quality improvement: remove an uneeded Obj.magic (#844, Benoit Montagu).
 
-  * On Windows, use the Unicode API in C stubs and functions introduced in OCaml 4.06 to handle Unicode strings. Raise the minimum requirement to OCaml 4.06 (on Windows only). (#843, Antonin Décimo)
+  * On Windows, use the Unicode API in C stubs and functions introduced in OCaml 4.06 to handle Unicode strings. Raise the minimum requirement to OCaml 4.06 (on Windows only). (#843, #903, Antonin Décimo)
 
 ===== 5.4.2 =====
 

src/unix/lwt_process_stubs.c

@@ -41,6 +41,7 @@ CAMLprim value lwt_process_create_process(value prog, value cmdline, value env,
 
   STARTUPINFO si;
   PROCESS_INFORMATION pi;
+  DWORD flags = CREATE_UNICODE_ENVIRONMENT;
   BOOL ret;
 
 #define string_option(opt) \
@@ -62,7 +63,7 @@ CAMLprim value lwt_process_create_process(value prog, value cmdline, value env,
   si.hStdOutput = get_handle(Field(fds, 1));
   si.hStdError = get_handle(Field(fds, 2));
 
-  ret = CreateProcess(progs, cmdlines, NULL, NULL, TRUE, 0,
+  ret = CreateProcess(progs, cmdlines, NULL, NULL, TRUE, flags,
                       envs, cwds, &si, &pi);
   caml_stat_free(progs);
   caml_stat_free(cmdlines);