Add --enable-cxx11 configure option

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...).

docs/changes.txt

@@ -81,6 +81,7 @@ Unix:
 
 - Fix bug in wxExecute() if child exited too quickly (Kevin B. McCarty).
 - Add --disable-sys-libs configure option.
+- Add --enable-cxx11 configure option.
 
 All (GUI):