Added -std=gnu++11 flag to atmelsam

[klenov]

Hello! Thanks for a splendid project!

At this moment the following code will not compile for Due or Zero boards. But the same code will compile in Arduino IDE (1.6.8).

void setup() {
  char *foo = nullptr; // check if c++11 is available
}

void loop() {
}

I'm not sure if my change is not in conflict with something else. There is also the same flag for Zero in arduino.py:266
https://github.com/platformio/platformio/blob/develop/platformio/builder/scripts/frameworks/arduino.py#L266

[ivankravets]

Thanks a lot!

@valeros please CHECK all flags between PlatformIO development platforms and Arduino IDE.