Fix memset_s() compilation errors under macOS with some SDKs

Define __STDC_WANT_LIB_EXT1__ as early as possible to ensure that it's
defined before string.h is included by some other header without it.

Closes #19334.

src/unix/utilsunx.cpp

@@ -18,6 +18,10 @@
 // for compilers that support precompilation, includes "wx.h".
 #include "wx/wxprec.h"
 
+// Define this as soon as possible and before string.h is included to get
+// memset_s() declaration from it if available.
+#define __STDC_WANT_LIB_EXT1__ 1
+
 #include "wx/utils.h"
 
 #if !defined(HAVE_SETENV) && defined(HAVE_PUTENV)
@@ -113,8 +117,6 @@
     #define _LANGUAGE_C_PLUS_PLUS 1
 #endif // SGI hack
 
-#define __STDC_WANT_LIB_EXT1__ 1 // for memset_s() in <string.h>
-
 #include <stdarg.h>
 #include <dirent.h>
 #include <string.h>