New(!?) warning makes configure for 21.4 break

Mats Lidell writes:

 > This is, or will be, a show stopper for 21.4 on Gentoo at
 > least. What is the right thing to do here? Go over our code and get
 > rid of or replace the defines according to what the warning
 > suggests or make the configure check more robust when it comes to
 > warnings or maybe both?

I'm not sure we should do anything (Vin really wants to stop
maintaining 21.4), but if we do my vote is +1 for fixing the configure
checks and -1 on the code changes.  It's not clear that those changes
would be trivial, especially if we have any code that does something
different #ifdef _BSD_SOURCE than for #ifdef _SVID_SOURCE.

But we may not have a choice, as configure.in is long obsolete, and I
suspect many distributions may stop supporting autoconf 2.13 in the
nearish future.

Steve