These flags have been discussed on another threadviewtopic.php?t=1241
It sounds like they are now the standard for the ARM Arduino build settings, so we should consider using them.
However I can only do limited testing e.g. with my projects, so the proof of the pudding would be just to update the repo and wait for people to complain if it caused side effects
Roger, I would recommend to open a post for the change and ask for people to test it.
I was in fact almost going to use the c11 flag the other day, but decided not to because if something worked for me with that may not work later with the normal repo, so I for one am happy to test it with a few things.
I'm using sdfat, an spi disaply, and the TMRpcm library I half finished to test the modifications to the SPI library, so I am using a few peripherals (spi1 and 2, DMA, Hardware Timers, Hardware Serial). If a few people joins using some more peripherals, we should be covered.