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/graph/graphml.hpp:41:25: warning: ‘virtual const char* boost::parse_error::what() const’ can be marked override [-Wsuggest-override]
./boost/graph/graphviz.hpp:622:23: warning: ‘virtual const char* boost::graph_exception::what() const’ can be marked override [-Wsuggest-override]
./boost/graph/graphviz.hpp:633:15: warning: ‘virtual const char* boost::bad_parallel_edge::what() const’ can be marked override [-Wsuggest-override]
./boost/graph/graphviz.hpp:645:23: warning: ‘virtual const char* boost::directed_graph_error::what() const’ can be marked override [-Wsuggest-override]
./boost/graph/graphviz.hpp:654:23: warning: ‘virtual const char* boost::undirected_graph_error::what() const’ can be marked override [-Wsuggest-override]
./boost/graph/graphviz.hpp:665:15: warning: ‘virtual const char* boost::bad_graphviz_syntax::what() const’ can be marked override [-Wsuggest-override]

BOOST_OVERRIDE was introduced in https://github.com/boostorg/config/commit/ffe4e0f5a448578cce14e3eed0cf7163333a81d9.

[EugeneZelenko]

See fixes in #217.

[hdu-sdlzx]

Why not close this issue? @EugeneZelenko @jzmaddock