use system's gf2x if available

<div id="comment:0"></div>

the usual spkg-configure.m4 thing

CC:  @embray @kiwifb

Component: **packages: standard**

Author: **Dima Pasechnik**

Branch: **[`844e241`](https://github.com/sagemath/sagetrac-mirror/commit/844e24113ea61ee3a6fd51c8119e32cb3e3ed40d)**

Reviewer: **François Bissey**

_Issue created by migration from https://trac.sagemath.org/ticket/27238_

