This issue has been reported here regularly here since the release of Arduino AVR Boards 1.6.12 (bundled with Arduino IDE 1.6.10). It is caused by an LTO (link time optimization) plugin added in that version. I don't believe the cause has ever been found. I can only offer some workarounds:
Option 1: Follow these instructions to revert to the pre-LTO Arduino AVR Boards package:
- Tools > Board > Boards Manager
- Wait for downloads to finish
- Click on "Arduino AVR Boards"
- Select 1.6.11 from the "select version" drop down menu.
- Click the "Install" button.
- Wait for installation to finish.
- Click the "Close" button.
OR
Option 2: Use the Arduino Web Editor:
Arduino Cloud.
More information here:
Arduino Project Hub