Feature/ignore unused

boost::ignore_unused_variable_warning() function is defined in ConceptCheck but also in the internals of other libraries (GIL, Interval, Test, uBlas, Unordered) probably to not include the whole concept_check.hpp. It would be convenient to have such tool ready-to-use in all libraries in a lightweight form, not introducing any additional dependencies. Newly introduced Boost.Core is a good place where it could reside.

Followed by the: https://github.com/boostorg/utility/pull/8
Discussion: http://boost.2283326.n4.nabble.com/Standalone-boost-ignore-unused-variable-warning-td4662481.html