Add Unix-like system support in octavelibczi.c

Introduce conditional compilation for non-Windows platforms by
defining _GNU_SOURCE and including necessary headers (dlfcn.h,
limits.h, stdlib.h, unistd.h) when _WIN32 is not defined. This
enhances compatibility and functionality for Unix-like systems.

OctaveMex/octavelibczi.c

@@ -5,6 +5,7 @@
 #ifdef _WIN32
 #include <Windows.h>
 #else
+#define _GNU_SOURCE
 #include <dlfcn.h>
 #include <limits.h>
 #include <stdlib.h>