Cheaper than fcntl(F_SETLK). The same is done already on Windows, so
if that works, why not use it everywhere? (Of course, only if the
compiler supports this C11 feature.)
As a bonus, the code in this commit also works on C++ via C++11
std::atomic, just in case somebody adds some C++ code to the opcache
extension one day.
061fcdb0a5