Made the mem_cache_block lockfree#23

Merged

jzmaddock merged 4 commits intoboostorg:developfrom

Jan 10, 2016

Commits on Dec 9, 2015

Made the mem_cache_block lockfree
Yucheng Low
committed

Commits on Dec 15, 2015

Updated with platform checks
Yucheng Low
committed

Commits on Dec 31, 2015

Lockfree mem_block_cache only active if std::atomic is available
Yucheng Low
committed
restored missing header
Yucheng Low
committed