SPISettings was not declared in this scope
If fixed this by deleting this built-in library

/usr/share/arduino/libraries/SD


but then I received the error:

/usr/share/arduino/libraries/Robot_Control/ArduinoRobot.cpp:25:2: error: 'Serial1' was not declared in this scope
  Serial1.begin(9600);


so I deleted these libraries as well:

/usr/share/arduino/libraries/Robot_Control
/usr/share/arduino/libraries/Robot_Motor


which appear to be buggy in the 1.6 Arduino release. It's still unclear why any of these libraries were being used, since I wasn't including any of their files.