Broken build on newer versions of LLVM

[Emil Gilliam <egilliam@…>]

Starting with LLVM r331746, the use of compare-and-swap on a const object is not allowed. On x86 and Clang, this breaks several places in atomic/detail/ops_gcc_x86_dcas.hpp, like this:

if defined(__clang__)
            // Clang cannot allocate eax:edx register pairs but it has sync intrinsics
            value = __sync_val_compare_and_swap(&storage, (storage_type)0, (storage_type)0);
#elif defined(BOOST_ATOMIC_DETAIL_X86_NO_ASM_AX_DX_PAIRS)
...

The use of compare-and-swap is actually a workaround for a Clang limitation which hasn't existed for a long time. Clang versions newer than 4.0.1 understand "=&A" contraints. I am attaching a proposed patch to ops_css_x86_dcas.hpp and config.hpp which detects the Clang version (using a has_feature), and if it's new enough, uses the "=&A" constraint, otherwise falls back to the existing compare-and-swap. (I had to do a bit of fancy footwork with the #ifs to make this happen.)

[Emil Gilliam <egilliam@…>]

Patch to Boost to fix compare-and-swap of const object in newer LLVM

[Emil Gilliam <egilliam@…>]

IGNORE OTHER ATTACHMENT - file paths are corrected in this patch.