script: fix compilation on GNU/Hurd (patch #7977) (thanks to Andrew Potter)

PATH_MAX is now defined in weechat-plugin.h (if not defined, for example on
GNU/Hurd), so that all plugins can use it.

src/plugins/weechat-plugin.h

@@ -24,6 +24,11 @@
 
 #include <sys/types.h>
 
+/* some systems like GNU/Hurd do not define PATH_MAX */
+#ifndef PATH_MAX
+    #define PATH_MAX 4096
+#endif
+
 struct t_config_option;
 struct t_gui_window;
 struct t_gui_buffer;