GCC suggest-override warnings

[EugeneZelenko]

Complete list of warnings when Boost 1.72 is built with GCC 7.5 with -Wsuggest-override added to cxxflags:

./boost/contract/core/exception.hpp:107:25: warning: ‘virtual const char* boost::contract::bad_virtual_result_cast::what() const’ can be marked override [-Wsuggest-override]
./boost/contract/core/exception.hpp:183:25: warning: ‘virtual const char* boost::contract::assertion_failure::what() const’ can be marked override [-Wsuggest-override]

BOOST_OVERRIDE was introduced in boostorg/config@ffe4e0f.

[lcaminiti]

This can be fixed using BOOST_OVERRIDE that should become part of Boost.Config from the next release: boostorg/config#329.
However, I will not generate a new release or Boost.Contract only to fix warnings. I will leave this issue open. If I have to generate a new Boost.Contract release in the future, I will then fix this warning as well.

[EugeneZelenko]

See fixes in #28.