The gphelp executable is needed by cypari2 at installation time.

It is provided by the separate package pari-doc on Debian.

This causes build errors on ubuntu-eoan-standard (https://github.com/mkoeppe/sage/runs/502310821?check_suite_focus=true)

CC: @dimpase @embray @kiwifb @isuruf @timokau @antonio-rojas

Component: build: configure

Author: Matthias Koeppe

Branch: 64a028d

Reviewer: Dima Pasechnik

Issue created by migration from https://trac.sagemath.org/ticket/29319