Hi,
I'm using arduino and platformio and I have the same problem.
I'll talk about arduino 1.8.7.
I have installed from https://github.com/stm32duino/BoardManagerFiles/raw/master/STM32/package_stm_index.json
the package "STM32 Cores by STMMicroelectronics version 1.5.0" for board "BluePill F103C8"
I have installed the library "MySensors by The MySensors Team Version 2.3.1"
And I have this error

Build options changed, rebuilding all
In file included from I:\ARDUINO\ARDUINO-1.8.5\OTO-DOCUMENTS\libraries\MySensors/hal/architecture/STM32F1/MyHwSTM32F1.cpp:20:0,
from I:\ARDUINO\ARDUINO-1.8.5\OTO-DOCUMENTS\libraries\MySensors/MySensors.h:69,
from C:\Users\oto\AppData\Local\Temp\arduino_modified_sketch_62106\sketch_feb21a.ino:44:
I:\ARDUINO\ARDUINO-1.8.5\OTO-DOCUMENTS\libraries\MySensors/hal/architecture/STM32F1/MyHwSTM32F1.h:23:27: fatal error: libmaple/iwdg.h: No such file or directory

 #include <libmaple/iwdg.h>

compilation terminated.

exit status 1
Error compiling for board Generic STM32F1 series.

Can anyone help me ?
Thanks !