You probably won’t find a file like that with all the -D flags listed out since they are all sourced from various makefiles, but in order to produce the list above I did the following in Python:
output = """
Building cpp file: /home/nrobinson/code/neopo/test//src/test.cpp
Invoking: ARM GCC CPP Compiler
mkdir -p ../build/target/user/platform-12-m/test/src/
arm-none-eabi-gcc -DSTM32_DEVICE -DnRF52840 -DNRF52840_XXAA -DPLATFORM_THREADING=1 -DPLATFORM_ID=12 -DPLATFORM_NAME=argon -DPLATFORM_GEN=3 -DUSBD_VID_SPARK=0x2B04 -DUSBD_PID_DFU=0xD00C -DUSBD_PID_CDC=0xC00C -DSPARK_PLATFORM -g3 -gdwarf-2 -Os -mcpu=cortex-m4 -mthumb -mabi=aapcs -mfloat-abi=hard -mfpu=fpv4-sp-d16 --specs=nano.specs -DINCLUDE_PLATFORM=1 -DSOFTDEVICE_PRESENT=1 -DS140 -DPRODUCT_ID=12 -DPRODUCT_FIRMWARE_VERSION=65535 -D_WIZCHIP_=W5500 -DUSE_STDPERIPH_DRIVER -DDFU_BUILD_ENABLE -DLFS_CONFIG=lfs_config.h -DSYSTEM_VERSION_STRING=3.0.0-beta.1 -DRELEASE_BUILD -I./inc -I../wiring/inc -I../system/inc -I../services/inc -I../third_party/nanopb/nanopb -I../communication/inc -I../hal/inc -I../hal/shared -I../hal/src/argon -I../hal/network/lwip/esp32 -I../hal/network/ncp/wifi -I../hal/src/nRF52840 -I../hal/src/armv7 -I../hal/src/nRF52840 -I../hal/src/nRF52840/lwip -I../hal/src/nRF52840/freertos -I../hal/src/nRF52840/mbedtls -I../hal/src/nRF52840/littlefs -I../hal -I../hal/src/nRF52840/posix -I../hal/network -I../hal/network/api -I../hal/network/lwip -I../hal/network/lwip/posix -I../hal/network/lwip/wiznet -I../hal/network/ncp -I../hal/network/ncp/at_parser -I../hal/network/util -I../third_party/lwip/lwip/src/include -I../third_party/freertos/freertos/FreeRTOS/Source/include -I../third_party/nrf5_sdk/nrf5_sdk/external/freertos/portable/GCC/nrf52 -I../third_party/nrf5_sdk/nrf5_sdk/external/freertos/portable/CMSIS/nrf52 -I../third_party/wiznet_driver/wiznet_driver/Ethernet -I../gsm0710muxer/gsm0710muxer/include -I../platform/shared/inc -I../third_party/nrf5_sdk -I../third_party/nrf5_sdk/nrf5_sdk/components/toolchain/cmsis/include -I../third_party/nrf5_sdk/nrf5_sdk/modules/nrfx -I../third_party/nrf5_sdk/nrf5_sdk/modules/nrfx/drivers -I../third_party/nrf5_sdk/nrf5_sdk/modules/nrfx/drivers/include -I../third_party/nrf5_sdk/nrf5_sdk/modules/nrfx/hal -I../third_party/nrf5_sdk/nrf5_sdk/modules/nrfx/mdk -I../third_party/nrf5_sdk/nrf5_sdk/modules/nrfx/hal -I../third_party/nrf5_sdk/nrf5_sdk/modules/nrfx/drivers/src/prs -I../third_party/nrf5_sdk/nrf5_sdk/components/libraries/util -I../third_party/nrf5_sdk/nrf5_sdk/components/softdevice/s140/headers -I../third_party/nrf5_sdk/nrf5_sdk/components/softdevice/s140/headers/nrf52 -I../third_party/nrf5_sdk/nrf5_sdk/components/softdevice/common -I../third_party/nrf5_sdk/nrf5_sdk/components/softdevice/mbr/nrf52840/headers -I../third_party/nrf5_sdk/nrf5_sdk/components/libraries/fstorage -I../third_party/nrf5_sdk/nrf5_sdk/components/libraries/experimental_section_vars -I../third_party/nrf5_sdk/nrf5_sdk/components/libraries/atomic -I../third_party/nrf5_sdk/nrf5_sdk/components/libraries/crc32 -I../third_party/nrf5_sdk/nrf5_sdk/integration/nrfx/legacy -I../third_party/nrf5_sdk/nrf5_sdk/components/libraries/delay -I../third_party/nrf5_sdk/nrf5_sdk/components/libraries/log -I../third_party/nrf5_sdk/nrf5_sdk/components/libraries/log -I../third_party/nrf5_sdk/nrf5_sdk/components/libraries/log/src -I../third_party/nrf5_sdk/nrf5_sdk/components/libraries/experimental_section_vars -I../third_party/nrf5_sdk/nrf5_sdk/components/libraries/atomic -I../third_party/nrf5_sdk/nrf5_sdk/components/libraries/delay -I../third_party/nrf5_sdk/nrf5_sdk/components/libraries/queue -I../third_party/nrf5_sdk/nrf5_sdk/components/softdevice/common -I../third_party/nrf5_sdk/nrf5_sdk/components/softdevice/s140 -I../third_party/nrf5_sdk/nrf5_sdk/components/softdevice/s140/headers -I../third_party/nrf5_sdk/nrf5_sdk/components/softdevice/s140/headers/nrf52 -I../third_party/nrf5_sdk/nrf5_sdk/components/libraries/fifo -I../third_party/nrf5_sdk/nrf5_sdk/components/libraries/atomic_fifo -I../third_party/nrf5_sdk/nrf5_sdk/components/libraries/strerror -I../third_party/nrf5_sdk/nrf5_sdk/components/libraries/fstorage -I../third_party/nrf5_sdk/nrf5_sdk/components/libraries/atomic_flags -I../third_party/nrf5_sdk/nrf5_sdk/components/libraries/usbd -I../third_party/nrf5_sdk/nrf5_sdk/components/libraries/usbd/class/cdc -I../third_party/nrf5_sdk/nrf5_sdk/components/libraries/usbd/class/cdc/acm -I../third_party/nrf5_sdk/nrf5_sdk/components/ble/common -I../third_party/nrf5_sdk/nrf5_sdk/components/ble/nrf_ble_gatt -I../third_party/nrf5_sdk/nrf5_sdk/components/ble/ble_advertising -I../third_party/nrf5_sdk/nrf5_sdk/components/nfc/t2t_lib -I../third_party/nrf5_sdk/nrf5_sdk/components/nfc/platform -I../third_party/nrf5_sdk/nrf5_sdk/components/nfc/ndef/generic/message -I../third_party/nrf5_sdk/nrf5_sdk/components/nfc/ndef/generic/record -I../third_party/nrf5_sdk/nrf5_sdk/components/nfc/ndef/lauchapp -I../third_party/nrf5_sdk/nrf5_sdk/components/nfc/ndef/text -I../third_party/nrf5_sdk/nrf5_sdk/components/nfc/ndef/uri -I../third_party/nrf5_sdk/nrf5_sdk/external/nrf_cc310/include -I../third_party/nrf5_sdk/nrf5_sdk/external/utf_converter -I../third_party/littlefs/littlefs -I../third_party/miniz/miniz -I../platform/MCU/nRF52840/inc -I../dynalib/inc -I/home/nrobinson/code/neopo/test//src -I./libraries -I. -MD -MP -MF ../build/target/user/platform-12-m/test/src/test.o.d -ffunction-sections -fdata-sections -Wall -Wno-switch -Wno-error=deprecated-declarations -fmessage-length=0 -fno-strict-aliasing -DSPARK=1 -DPARTICLE=1 -Wundef -DSTART_DFU_FLASHER_SERIAL_SPEED=14400 -DSTART_YMODEM_FLASHER_SERIAL_SPEED=28800 -DSPARK_PLATFORM_NET=ESP32 -fno-builtin-malloc -fno-builtin-free -fno-builtin-realloc -DLOG_INCLUDE_SOURCE_INFO=1 -DPARTICLE_USER_MODULE -DUSER_FIRMWARE_IMAGE_SIZE=0x20000 -DUSER_FIRMWARE_IMAGE_LOCATION=0xD4000 -DMODULAR_FIRMWARE=1 -DMODULE_VERSION=6 -DMODULE_FUNCTION=5 -DMODULE_INDEX=1 -DMODULE_DEPENDENCY=4,1,3002 -DMODULE_DEPENDENCY2=0,0,0 -D_GNU_SOURCE -DLOG_MODULE_CATEGORY="\"app\"" -fno-exceptions -fno-rtti -fcheck-new -flto -ffat-lto-objects -DPARTICLE_COMPILE_LTO_FAT -std=gnu++14 -c -o ../build/target/user/platform-12-m/test/src/test.o /home/nrobinson/code/neopo/test//src/test.cpp
"""
gcc_line = output.splitlines()[-1].rstrip()
d_flags = [word.split("-D")[-1] for word in gcc_line.split() if word.startswith("-D")]
print(*d_flags, sep="\n")
Output:
STM32_DEVICE
nRF52840
NRF52840_XXAA
PLATFORM_THREADING=1
PLATFORM_ID=12
PLATFORM_NAME=argon
PLATFORM_GEN=3
USBD_VID_SPARK=0x2B04
USBD_PID_DFU=0xD00C
USBD_PID_CDC=0xC00C
SPARK_PLATFORM
INCLUDE_PLATFORM=1
SOFTDEVICE_PRESENT=1
S140
PRODUCT_ID=12
PRODUCT_FIRMWARE_VERSION=65535
_WIZCHIP_=W5500
USE_STDPERIPH_DRIVER
DFU_BUILD_ENABLE
LFS_CONFIG=lfs_config.h
SYSTEM_VERSION_STRING=3.0.0-beta.1
RELEASE_BUILD
SPARK=1
PARTICLE=1
START_DFU_FLASHER_SERIAL_SPEED=14400
START_YMODEM_FLASHER_SERIAL_SPEED=28800
SPARK_PLATFORM_NET=ESP32
LOG_INCLUDE_SOURCE_INFO=1
PARTICLE_USER_MODULE
USER_FIRMWARE_IMAGE_SIZE=0x20000
USER_FIRMWARE_IMAGE_LOCATION=0xD4000
MODULAR_FIRMWARE=1
MODULE_VERSION=6
MODULE_FUNCTION=5
MODULE_INDEX=1
MODULE_DEPENDENCY=4,1,3002
MODULE_DEPENDENCY2=0,0,0
_GNU_SOURCE
LOG_MODULE_CATEGORY=""app""
PARTICLE_COMPILE_LTO_FAT