This is in preparation for adding support for a C++11 based parallel
for implementation. The code is behind CERES_USE_CXX11_THREADS which
is not exposed to the user yet.
Tested by building with and without CERES_USE_CXX11_THREADS defined
and the tests pass.
Change-Id: I60f5730fa055feeb0ee0fa6c980633aebd8d87b4