As a result of refactoring configure.ac in #24919,
absence of (g)fortran now forces installation of full gcc, instead of just gfortran part of it.

This is particularly crucial on OSX, where we don't normally build full gcc any more, but only build gfortran.

To reproduce, move gfortran out of your PATH, and re-run ./configure. You'll see that now gcc and gfortran are scheduled for installation...

CC: @embray @jhpalmieri @vbraun @kiwifb

Component: build: configure

Author: François Bissey

Branch: 3185e91

Reviewer: Erik Bray

Issue created by migration from https://trac.sagemath.org/ticket/26715