Fix build warnings/errs w/ -DNO_GETENV/-DNO_PUTENV

- strtest.c: Fix unused variable warnings if both -DNO_GETENV and
  -DNO_PUTENV are specified or if only -DNO_GETENV is specified.

- jinclude.h: Fix build error if only -DNO_GETENV is specified.

Fixes #697

jinclude.h

@@ -4,7 +4,7 @@
  * This file was part of the Independent JPEG Group's software:
  * Copyright (C) 1991-1994, Thomas G. Lane.
  * libjpeg-turbo Modifications:
- * Copyright (C) 2022, D. R. Commander.
+ * Copyright (C) 2022-2023, D. R. Commander.
  * For conditions of distribution and use, see the accompanying README.ijg
  * file.
  *
@@ -123,6 +123,8 @@ static INLINE int GETENV_S(char *buffer, size_t buffer_size, const char *name)
 
 #else
 
+#include <errno.h>
+
 /* This provides a similar interface to the Microsoft _putenv_s() function, but
  * other than parameter validation, it has no advantages over setenv().
  */

strtest.c

@@ -1,5 +1,5 @@
 /*
- * Copyright (C)2022 D. R. Commander.  All Rights Reserved.
+ * Copyright (C)2022-2023 D. R. Commander.  All Rights Reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are met:
@@ -55,8 +55,12 @@ void invalid_parameter_handler(const wchar_t *expression,
 
 int main(int argc, char **argv)
 {
+#if !defined(NO_GETENV) || !defined(NO_PUTENV)
   int err;
+#endif
+#ifndef NO_GETENV
   char env[3];
+#endif
 
 #ifdef _MSC_VER
   _set_invalid_parameter_handler(invalid_parameter_handler);