Compile error with #include <unistd.h>

IDE: Arduino 1.8.7 on Windows.
Core version: Arduino SAM Boards 1.6.11, installed from Boards Manager

MCVE sketch:
```
#include

void setup() {
}

void loop() {
}
```

Error message:
```
Arduino: 1.8.7 (Windows Store 1.8.15.0) (Windows 10), Board: "Arduino Due (Programming Port)"
In file included from c:\users\sunny\documents\arduinodata\packages\arduino\tools\arm-none-eabi-gcc\4.8.3-2014q1\arm-none-eabi\include\stdlib.h:11:0,
from C:\Users\sunny\Documents\ArduinoData\packages\arduino\hardware\sam\1.6.11\cores\arduino/Arduino.h:24,
from sketch\sketch_nov18a.ino.cpp:1:
c:\users\sunny\documents\arduinodata\packages\arduino\tools\arm-none-eabi-gcc\4.8.3-2014q1\arm-none-eabi\include\sys\unistd.h:118:9: error: declaration of C function 'int link(const char*, const char*)' conflicts with
int _EXFUN(link, (const char *__path1, const char *__path2 ));
^
In file included from C:\Users\sunny\Documents\ArduinoData\packages\arduino\hardware\sam\1.6.11\variants\arduino_due_x/variant.h:50:0,
from C:\Users\sunny\Documents\ArduinoData\packages\arduino\hardware\sam\1.6.11\cores\arduino/Arduino.h:201,
from sketch\sketch_nov18a.ino.cpp:1:
C:\Users\sunny\Documents\ArduinoData\packages\arduino\hardware\sam\1.6.11\cores\arduino/syscalls.h:43:12: error: previous declaration 'int link(char*, char*)' here
extern int link( char *cOld, char *cNew ) ;
^
exit status 1
Error compiling for board Arduino Due (Programming Port).
```

The header `unistd.h` is used by [ArduinoCbor](https://github.com/bergos/ArduinoCbor/) library in [cn-cbor.h](https://github.com/bergos/ArduinoCbor/blob/c142cb0cd650e07ef069448b09c27c994c346381/src/cn-cbor/cn-cbor.h#L19).

Start with cores/arduino/syscalls.h and variants/arduino_due_x/variant.h, then inspect the unistd.h inclusion from ArduinoCbor's src/cn-cbor/cn-cbor.h. Reproduce the MCVE on an Arduino Due with Arduino SAM Boards 1.6.11 and confirm that the include no longer produces conflicting link declarations.

Written by the indexing model from the issue text.