Another round of attempting to squelch -Wdeprecated-declarations, which
has become very trigger-happy with libc++ 9.0.0.
It does not help that gcc's implementation of this warning is even more
trigger-happy, in the sense that it already warns on the declaration
itself, not when you are using it. This is very annoying with our use
of -Wsystem-headers. That should really be disabled for gcc.