#if BUILT_VARIANT=1
    #define MAX_QSIZE 100
    #define RELAY_COUNT 4
    #define HAVE_UART2
    ...
#elif BUILD_VARIANT=2
    #define MAX_QSIZE 20
    #define RELAY_COUNT 8
    #define HAVE_SPI
    #define HAVE_BATTERY_CHARGER
    ...
#else
    #error "Unsupported build alternative."
#endif