(from #31348)

... using SAGE_NTL_PREFIX via sage_conf.

This is for macOS with configurations in which Python extensions no longer have access to /usr/local due to the use of -isysroot in the compiler configuration from sysconfig.

In particular, we add handling for ntl to the .homebrew-build-env script -- so that after brew install ntl; brew unlink ntl, our ./configure still finds NTL.

Depends on #30770
Depends on #31344

CC: @zlscherr @dimpase @orlitzky @videlec @kliem @jhpalmieri @kiwifb

Component: build: configure

Author: Matthias Koeppe

Branch/Commit: dbcbf79

Reviewer: Jonathan Kliem

Issue created by migration from https://trac.sagemath.org/ticket/31365