The proposed define is for satisfying the Device object optionality from footnotes:

...
³If the device supports the execution of the TimeSynchronization service, then these properties shall be present.
⁴If the device supports the execution of the UTCTimeSynchronization service, then these properties shall be present.
¹²These properties shall be present if the device is capable of tracking date and time.

To support the optionality, 3 defines should be used.

Which device.c do you want to change? src/bacnet/basic/object/device.c or src/bacnet/basic/server/bacnet_device.c or all the device.c examples?

Since the device object is probably the most customized object, I started to refactor the mechanics of the BACnet device object into a handler - see the branch refactor-device-object-handling - to make the 'custom' properties and objects an easier task - create the object table and local device service handlers which can call into the handler_device_xyz() for properties and services that the handler handles.

I also implemented the src/bacnet/basic/server/bacnet_device.c which has #define all throughout and is used by BACnet on Zephyr OS where the optionality is configured through the CONFIG_BACNET_xyz defines for constrained microcontrollers, and the example apps/server-basic uses this device object example.

I used another technique with the src/bacnet/basic/object/netport.c to reduce the amount of #ifdef clutter. I mostly used checks for protocol revision (of course, Network Ports object is rife with protocol-revision specific changes) but only modify the property lists.

static const int Ethernet_Port_Properties_Optional[] = {
    /* unordered list of optional properties */
    PROP_DESCRIPTION,
    PROP_MAC_ADDRESS,
#if (BACNET_PROTOCOL_REVISION >= 24)
    PROP_APDU_LENGTH,
    PROP_NETWORK_NUMBER,
    PROP_NETWORK_NUMBER_QUALITY,
    PROP_LINK_SPEED,
#endif
    -1
};

The read/write property handlers include all the possible properties, but check for property membership before processing:

    if (!Property_List_Member(
            wp_data->object_instance, wp_data->object_property)) {
        wp_data->error_class = ERROR_CLASS_PROPERTY;
        wp_data->error_code = ERROR_CODE_UNKNOWN_PROPERTY;
        return false;
    }

Of course, this technique will not result in smaller firmware but is intended for the OS based (Linux/Window/BSD/macOS/Zephyr) BACnet applications.

Note that the #define namespace becomes increasingly important in these larger integrated projects. The define name should follow the variable and function name coding standard:

Variable and function names are defined with the first words being
descriptive of broad ideas, and later words narrowing down to specifics.
For instance: Universe_Galaxy_System_Planet. 

There are some existing BACNET_SVC_TS_A config.h defines, but I would prefer that we use BIBB nomenclature for new names (and maybe we update the SVC names to BIBB and deprecate SVC in the future). The 3 Time Service related defines could be:

#define BACNET_BIBB_DM_TS_B_ENABLED
#define BACNET_BIBB_DM_UTC_B_ENABLED
#define BACNET_DATE_TIME_TRACKING_ENABLED

Thoughts?