See https://github.com/arduino/Arduino/pull/2175

Quoting the original commits from Matthijs Kooijman:

    Enable C++11 support

    This uses the gnu++11 standard, which is C++11 with GNU extensions.
    C++11 should be full compatible with the previously used C++98
    standards, so all pre-existing sketches should continue to work.

    Enable C11 support

    This uses the gnu11 standard, which is C11 with GNU extensions.
    Previously, gnu89 was being used, which is pretty ancient by now. C99
    brings some important improvements, some of which were already available
    and used even without this option. C11 is more recent and brings more
    minor improvements. Most notable feature is the static_assert statement,
    allowing checking invariants at compiletime using the full C
    expressions.