Use a different set of macros and types for atomics when compiled in C++
Use a different set of macros and types for atomics when compiled in C++

    Previously, the `atomics.h` header relied solely on the presence of
    the C11 specific header `stdatomic.h`. Now, when compiled in C++,
    we use the `std::atomic` type family provided by the STL in C++11.

andrewrk/libsoundio,tresf/libsoundio,andrewrk/libsoundio,tresf/libsoundio