This ticket adds an spkg-configure.m4 for zn_poly, in order to use it from a system package if possible (see #27330).

Nothing special with this one: Sage builds without any problem with this system package.

CC: @orlitzky @kiwifb @mkoeppe @isuruf

Component: build: configure

Keywords: zn_poly, system packages

Author: Thierry Thomas, Dima Pasechnik

Branch/Commit: 9201dde

Reviewer: Matthias Koeppe

Issue created by migration from https://trac.sagemath.org/ticket/29649