bug#50058: 28.0.5; emacs build fails with undefined reference to malloc_info

[Eli Zaretskii]

> From: Lars Ingebrigtsen <larsi@gnus.org>
> Cc: birdsite@airmail.cc,  50058@debbugs.gnu.org
> Date: Sat, 14 Aug 2021 19:12:37 +0200
> 
> I think musl, on purpose, doesn't define anything to allow us to
> identify that we're using it? 
> 
> https://wiki.musl-libc.org/faq.html
> 
> Or I may be misunderstanding something.
> 
> But if that's the case, we have to just test explicitly for malloc_info
> in configure.ac...

Yes.  Or we could use something to single out glibc, if there is such
a macro.