//These 3 defines are only here to show an example
//They are normally defined in the build configuration for each customer
//So not everything exists normally
#define CONFIG_CUSTOMER1
//#define CONFIG_CUSTOMER2
//#define CONFIG_CUSTOMER3

#ifdef CONFIG_CUSTOMER1
    #define WITH_METER
    #define WITH_DISPLAY
#elif defined(CONFIG_CUSTOMER2)
    #define WITH_METER
    #define WITH_SCOPE
#elif define(CONFIG_CUSTOMR3)
    #define WITH_DISPLAY
    #define WITH_GPS
#else
    #error "ABC"
#endif

void some_function() {
    //Do some stuff
    #ifdef WITH_GPS
        //Do stuff if GPS
    #endif
    //Do rest stuff
}