build: Use __register_atfork() only if really available

build: Use __register_atfork() only if really available

This is a glibc-specific symbol that has no public declaration. But is
being used by the OpenBSD and this implementation as a hack to avoid
having to link against the pthread library. This interface is at least
included in LSB 5.0 [L], and using pthread_atfork() is otherwise
problematic anyway [P].

 [L] <https://refspecs.linuxfoundation.org/LSB_5.0.0/LSB-Core-generic/LSB-Core-generic/baselib---register-atfork.html>
 [P] <http://austingroupbugs.net/view.php?id=851>

One problem is that we were using it whenever __GLIBC__ is defined,
which is supposed to be defined only on an actual glibc, but uClibc
defines that macro, but it does not provide the symbol on its noMMU
variant.

We add a new configure check that will try to link a program that uses
that symbol to make sure it is present.

Closes: !2
Reported-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>