Use the latest version of ax_cxx_compile_stdcxx.m4 from the autoconf
archive for C++17 support and handle "17" as the option value in our
configure.
See https://github.com/wxWidgets/wxWidgets/pull/721
Make it simpler to enable C++11 support when building wxWidgets, in particular
take care of using the correct standard library under OS X in this case.
Notice that currently we still build the library using C++98 if no C++11
compiler is available, even with this option. We may want to change this to
give an error in such case later instead.
Also skip the check for <type_traits> in configure when C++11 is used, we know
that it's available in this case, so don't waste time checking for it (there
are probably several more checks that could be skipped in C++11 mode too...).
