Fix GCC build errors

[qwertyuiopleowo]

When following the SDL build instructions for my linux machine, I encountered 2 types of compile errors running make.

This pull request fixes the errors by:

• suppressing a compile error by adding an additional C_FLAG in the CMakeLists.txt
• inserting function return type void for 2 functions

System

Architecture: AMD64 / x86_64
OS: Linux 6.17.9-arch1-1
Compiler: GCC 15.2.21 20251112

GCC error examples

NP2kai/cbus/cbuscore.c:66:25: error: initialization of ‘void (*)(const NP2CFG *)’ {aka ‘void (*)(const struct tagNP2Config *)’} from incompatible pointer type ‘void (*)(void)’ [-Wincompatible-pointer-types]
   66 |                         bmsio_reset,
      |                         ^~~~~~~~~~~

NP2kai/x/gtk2/gtk_menu.c:799:1: error: return type defaults to ‘int’ [-Wimplicit-int]
  799 | xmenu_visible_item(MENU_HDL hdl, const char *name, BOOL onoff)
      | ^~~~~~~~~~~~~~~~~~