Serial Uart programming with C

Hardware Raspberry Zero, Serial0
My Software was written for raspbian-strech-lite and raspian-buster-lite and works well.
It uses Hardware control RTSCTS. CTS to stop Transmit.
I now install an aktuall dietpi and translate the software
Output of data via serial0 is ok, but stop via CTS does not work.
program uses termios to activate RTS/CTS
struct termios options;
options.c_cflag |= CRTSCTS;// RTSCTS aktivate

in documentation CRTSCTS can only be used with _BSD_SOURCE
in dietpi _DEFAULT_SOURCE is used
what can I do?