On Unix, dbus has historically used gcc-specific lock-free atomic
intrinsics where available, falling back to a pthreads mutex where
possible. Meanwhile, on Windows, it has historically used
InterlockedIncrement() and similar library functions (in practice
wrappers around lock-free intrinsics on real Windows, but IPC calls into
wineserver on Wine).

ISO C11 provides a new header, stdatomic.h, with standardized support
for atomic operations. Exactly how these are implemented is a compiler
quality-of-implementation decision, but any reasonable compiler
implementation on a modern CPU should be using intrinsics. Let's use
this wherever possible, falling back to our old implementation only if
the C11 implementation is unsupported.

One concrete benefit that we get from this is that when compiling with
mingw-w64 gcc and running via Wine, this makes atomic reference counting
operations into a simple local operation, rather than IPC to wineserver
which can be very slow....