See discussion at
Homebrew/homebrew-core#53192

Instead of using gettextize at bootstrap time, we just vendor the files m4/iconv.m4 and config/config.rpath in the source tree.

This is done by using gnulib: gnulib-tool --import iconv (and ignoring all the other files that it is offering us).

CC: @dimpase

Component: build: configure

Author: Matthias Koeppe

Branch: d9bd946

Reviewer: Dima Pasechnik

Issue created by migration from https://trac.sagemath.org/ticket/29549