[boost] [lockfree] arm / iOS builds against Boost.Atomic

Hello, I noticed today when moving my work from OS X / clang / libc++ to iOS / clang / libc++ that Boost.Lockfree must link against libboost_atomic.a to compile. This shouldn't be necessary, since there is std::atomic on iOS as well. If this is correct I can file a bug report, provide a test application, or both if requested. cheers, Rich