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

Remark: it works as-is when pynac has been built without giac, i.e. if --without-giac has been set (when giac is installed, pynac finds and uses it by default). And if pynac has been built against giac, src/module_list.py must be patched.

Depends on #32386

CC: @kiwifb @orlitzky @dimpase

Component: build: configure

Keywords: spkg-configure pynac giac

Branch/Commit: public/29542 @ 1c424ad

Reviewer: Dima Pasechnik

Issue created by migration from https://trac.sagemath.org/ticket/29542