Try and build under PlatformIO

platformio.ini

@@ -0,0 +1,31 @@
+; PlatformIO Project Configuration File
+;
+;   Build options: build flags, source filter
+;   Upload options: custom upload port, speed and extra flags
+;   Library options: dependencies, extra library storages
+;   Advanced options: extra scripting
+;
+; Please visit documentation for the other options and examples
+; https://docs.platformio.org/page/projectconf.html
+
+[env:MMDVM]
+platform = ststm32
+board = genericSTM32F446RE
+framework = arduino, cmsis
+build_unflags = -O0
+monitor_speed = 460800
+build_flags = 
+    -O3
+    -Wall
+    ; -D ARM_MATH_CM7
+    ; -mfpu=fpv5-sp-d16
+    -D ARM_MATH_CM4
+    -mfpu=fpv4-sp-d16
+    -mfloat-abi=hard
+    ; enable USB serial
+    -D PIO_FRAMEWORK_ARDUINO_USB_HIGHSPEED_FULLMODE
+    -D PIO_FRAMEWORK_ARDUINO_ENABLE_CDC
+    -D USBCON
+    ; increase the buffer sizes
+    -D SERIAL_RX_BUFFER_SIZE=1024
+    -D SERIAL_TX_BUFFER_SIZE=1024