Just cast TCIOFLUSH to a pointer. The problem has been solved in the svn version of synaser:
{$IFNDEF MSWINDOWS}
procedure TBlockSerial.Purge;
begin
{$IFNDEF FPC}
SerialCheck(ioctl(FHandle, TCFLSH, TCIOFLUSH));
{$ELSE}
{$IFDEF DARWIN}
SerialCheck(fpioctl(FHandle, TCIOflush, Pointer(PtrInt(TCIOFLUSH))));
{$ELSE}
SerialCheck(fpioctl(FHandle, TCFLSH, Pointer(PtrInt(TCIOFLUSH))));
{$ENDIF}
{$ENDIF}
FBuffer := '';
ExceptCheck;
end;
{$ELSE}