[Bug libstdc++/59325] Provide a way to disable deprecated warnings

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59325

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Doing this before any other includes will work:

#include <bits/c++config>
#undef _GLIBCXX_DEPRECATED
#define _GLIBCXX_DEPRECATED

I'm not convinced we want to support this officially.