cmdutils: check for SetDllDirectory() availability

It's only available on Windows XP or newer.

Should fix compilation with mingw32 using the default OS target.

Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: James Almer <jamrial@gmail.com>

cmdutils.c

@@ -108,7 +108,7 @@ static void log_callback_report(void *ptr, int level, const char *fmt, va_list v
 
 void init_dynload(void)
 {
-#ifdef _WIN32
+#if HAVE_SETDLLDIRECTORY
     /* Calling SetDllDirectory with the empty string (but not NULL) removes the
      * current working directory from the DLL search path as a security pre-caution. */
     SetDllDirectory("");