Refactor includes and add NaN/Inf check function

Moved `#define _GNU_SOURCE` to the top of the file to ensure GNU-specific features are available before any includes. Reordered `#include` directives to avoid potential conflicts by placing them after platform-specific headers. Added a new static function `octaveMexIsNanOrInfDouble` to check if a double value is NaN or infinity using `mxIsNaN` and `mxIsInf`.

OctaveMex/octavelibczi.c

@@ -1,17 +1,18 @@
 #define MX_HAS_INTERLEAVED_COMPLEX 1
-#include "mex.h"
-#include "../AppModel/include/app_api.h"
-#include <string.h>
+#define _GNU_SOURCE
 #ifdef _WIN32
 #include <Windows.h>
 #else
-#define _GNU_SOURCE
 #include <dlfcn.h>
 #include <limits.h>
 #include <stdlib.h>
 #include <unistd.h>
 #endif
 
+#include "mex.h"
+#include "../AppModel/include/app_api.h"
+#include <string.h>
+
 static bool octaveMexIsNanOrInfDouble(double value)
 {
     return mxIsNaN(value) || mxIsInf(value);