Adding that build flag throws a new error, which looks less intimidating than the previous one, but still intimidating.
The terminal process terminated with exit code: 1
Terminal will be reused by tasks, press any key to close it.
> Executing task: platformio run --environment stm32f407vg <
Processing stm32f407vg (platform: ststm32; board: disco_f407vg)
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CONFIGURATION: https://docs.platformio.org/page/boards/ststm32/disco_f407vg.html
PLATFORM: ST STM32 5.7.0 > ST STM32F4DISCOVERY
HARDWARE: STM32F407VGT6 168MHz, 128KB RAM, 1MB Flash
DEBUG: Current (stlink) On-board (stlink) External (blackmagic, jlink)
PACKAGES: toolchain-gccarmnoneeabi 1.70201.0 (7.2.1)
LDF: Library Dependency Finder -> http://bit.ly/configure-pio-ldf
LDF Modes: Finder ~ chain, Compatibility ~ soft
Found 0 compatible libraries
Scanning dependencies...
No dependencies
arm-none-eabi-g++ -o .pio/build/stm32f407vg/src/main.o -c -fno-rtti -fno-exceptions -std=c++1z -std=gnu++1z -ffunction-sections -fdata-sections -Wall -mthumb -nostdlib -mcpu=cortex-m4 -fno-use-cxa-atexit -Og -g3 -ggdb3 -DF_CPU=168000000L -DDISCO_F407VG -DPLATFORMIO=40003 -DSTM32F4 -DSTM32F407xx -DSTM32F40_41xxx -DARDUINO_STM32DiscoveryF407 -DBOARD_discovery_f4 -DSTM32F407VG -DDEBUG -D__PLATFORMIO_BUILD_DEBUG__ -Iinclude -Isrc src/main.cpp
arm-none-eabi-g++ -o .pio/build/stm32f407vg/src/target/STM32F407VG/InterruptVectorTable.o -c -fno-rtti -fno-exceptions -std=c++1z -std=gnu++1z -ffunction-sections -fdata-sections -Wall -mthumb -nostdlib -mcpu=cortex-m4 -fno-use-cxa-atexit -Og -g3 -ggdb3 -DF_CPU=168000000L -DDISCO_F407VG -DPLATFORMIO=40003 -DSTM32F4 -DSTM32F407xx -DSTM32F40_41xxx -DARDUINO_STM32DiscoveryF407 -DBOARD_discovery_f4 -DSTM32F407VG -DDEBUG -D__PLATFORMIO_BUILD_DEBUG__ -Iinclude -Isrc src/target/STM32F407VG/InterruptVectorTable.cpp
arm-none-eabi-gcc -x assembler-with-cpp -ffunction-sections -fdata-sections -Wall -mthumb -nostdlib -mcpu=cortex-m4 -DF_CPU=168000000L -DDISCO_F407VG -DPLATFORMIO=40003 -DSTM32F4 -DSTM32F407xx -DSTM32F40_41xxx -DARDUINO_STM32DiscoveryF407 -DBOARD_discovery_f4 -DSTM32F407VG -DDEBUG -D__PLATFORMIO_BUILD_DEBUG__ -Iinclude -Isrc -c -o .pio/build/stm32f407vg/src/target/STM32F407VG/startup_stm32f4xx.o src/target/STM32F407VG/startup_stm32f4xx.S
arm-none-eabi-gcc -o .pio/build/stm32f407vg/src/target/STM32F407VG/system_stm32f4xx.o -c -ffunction-sections -fdata-sections -Wall -mthumb -nostdlib -mcpu=cortex-m4 -fno-use-cxa-atexit -Og -g3 -ggdb3 -DF_CPU=168000000L -DDISCO_F407VG -DPLATFORMIO=40003 -DSTM32F4 -DSTM32F407xx -DSTM32F40_41xxx -DARDUINO_STM32DiscoveryF407 -DBOARD_discovery_f4 -DSTM32F407VG -DDEBUG -D__PLATFORMIO_BUILD_DEBUG__ -Iinclude -Isrc src/target/STM32F407VG/system_stm32f4xx.c
In file included from include/hal/InterruptVectorTable.hpp:3:0,
from src/main.cpp:3:
include/config.hpp:6:35: note: #pragma message: 201703L
#pragma message (XSTR(__cplusplus))
^
cc1: warning: command line option '-fno-use-cxa-atexit' is valid for C++/ObjC++ but not for C
In file included from include/hal/InterruptVectorTable.hpp:3:0,
from src/target/STM32F407VG/InterruptVectorTable.cpp:2:
include/config.hpp:6:35: note: #pragma message: 201703L
#pragma message (XSTR(__cplusplus))
^
arm-none-eabi-g++ -o .pio/build/stm32f407vg/firmware.elf -T stm32f405x6.ld -Wl,--gc-sections,--relax -mthumb -nostartfiles -nostdlib -mcpu=cortex-m4 .pio/build/stm32f407vg/src/main.o .pio/build/stm32f407vg/src/target/STM32F407VG/InterruptVectorTable.o .pio/build/stm32f407vg/src/target/STM32F407VG/startup_stm32f4xx.o .pio/build/stm32f407vg/src/target/STM32F407VG/system_stm32f4xx.o -L/Users/john/.platformio/platforms/ststm32/ldscripts -L.pio/build/stm32f407vg -Wl,--start-group -lc -lgcc -lm -lstdc++ -lnosys -Wl,--end-group
/Users/john/.platformio/packages/toolchain-gccarmnoneeabi/bin/../lib/gcc/arm-none-eabi/7.2.1/../../../../arm-none-eabi/lib/thumb/v7e-m/libc.a(lib_a-fini.o): In function `__libc_fini_array':
fini.c:(.text.__libc_fini_array+0x26): undefined reference to `_fini'
collect2: error: ld returned 1 exit status
*** [.pio/build/stm32f407vg/firmware.elf] Error 1