What did I miss ? The file "platform.local.txt" is no longer used to add compiler options. I have the file next to the file "platform.txt" in: arduino-1.6.3 / hardware / arduino / avr But even the extra options (like compiler.c.extra_flags) in "platform.txt" itself are ignored. Running Ubuntu Linux 64-bit.
[UPDATE] It works when I place the "platform.local.txt" in this folder: .arduino15 / packages / arduino / hardware / avr / 1.6.2 Why does it work with the folder "1.6.2" when I'm running "1.6.3" ? I remove the "packages" folder and *.json files from the .arduino15 folder for a fresh start.