Fix undefined reference to `__atomic_fetch_add_8' (kimwalisch#46)

* Find libatomic

* Improve libatomic detection

* Use -std=c++11  to compile <atomic>

* Refactor libatomic detection

* Changes in version 6.3

* Use CMAKE_REQUIRED_FLAGS instead of CMAKE_REQUIRED_DEFINITIONS

Fix undefined reference to `__atomic_fetch_add_8' (kimwalisch#46)

* Find libatomic

* Improve libatomic detection

* Use -std=c++11  to compile <atomic>

* Refactor libatomic detection

* Changes in version 6.3

* Use CMAKE_REQUIRED_FLAGS instead of CMAKE_REQUIRED_DEFINITIONS

Rename threads to l2Sharing

Changes in version 6.1

Update release date

Fix set_num_threads () documentation

Refactor

Update RELEASE.md

Increase version to 5.7.1

Increase version to 5.7.0