make code compatible with up-to-date build environment

These changes make it possible to compile open9x with old (avr-gcc 4.3.3, 
avr-libc 1.6.7) and up-to-date (avr-gcc 4.6.2, avr-libc 1.8.0) build
environments. See Issue 1 for more details.

src/general_menus.cpp

@@ -44,7 +44,7 @@ void menuProcDiagKeys(uint8_t event);
 void menuProcDiagAna(uint8_t event);
 void menuProcDiagCalib(uint8_t event);
 
-MenuFuncP_PROGMEM APM menuTabDiag[] = {
+const MenuFuncP_PROGMEM menuTabDiag[] PROGMEM = {
   menuProcSetup,
 #if defined(PCBV4)
   menuProcTime,