Fixing build with glibc 2.34.

Explicitly using the sysconf() call to obtain the minimum thread stack size
instead of the PTHREAD_STACK_MIN macro.

This closes #576 PR on GitHub.

docs/changes.xml

@@ -31,6 +31,12 @@ NGINX Unit updated to 1.26.0.
          date="" time=""
          packager="Andrei Belov <defan@nginx.com>">
 
+<change type="bugfix">
+<para>
+fixed building with glibc 2.34, notably Fedora 35.
+</para>
+</change>
+
 </changes>