Reduce compiler warnings when boost uses std-atomics

Reduce compiler warnings when boost uses std-atomics

This works around for compilers with non-static-data-member
initialization.

spinlock_t is-a struct { lockType _; } and BOOST_DETAIL_SPINLOCK_INIT
initializes the first member of the struct.
All defines of BOOST_DETAIL_SPINLOCK_INIT include c-style curly braces
to initialize the struct's data member.

However, modern C++ compiler interpret the braces differently resulting
in copy constriction of the initializer.