Add ignore_unused_variable_warning()

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. This pull request adds the tool to the Utility and expands its functionality by the use of variadic template (if supported) or overloads.
Followed by: http://boost.2283326.n4.nabble.com/Standalone-boost-ignore-unused-variable-warning-td4662481.html