Provide precious variables (CFLAGS, PKG_CONFIG_PATH, ...) to make #29507
Description
autotools have a concept of "precious variables"
(https://www.gnu.org/software/autoconf/manual/autoconf-2.69/html_node/Setting-Output-Variables.html).
Because we do not use automake, many standard precious variables such as CFLAGS, PKG_CONFIG_PATH are not actually applied to building.
This should be fixed to make our build system more standard.
(from discussions in #29504, and a discussion with gh-mwageringel on a ticket I lost track of)
Likewise, for variables set by .homebrew-build-env (https://groups.google.com/d/msg/sage-devel/taOGXzXq6Lk/ePcXsCvhBAAJ)
See also:
- Make SAGE* environment variables set at configure time to stick #23570 Make SAGE* environment variables set at configure time to stick
CC: @jhpalmieri @mwageringel @dimpase @kiwifb @kliem @dcoudert
Component: build: configure
Keywords: sd111
Issue created by migration from https://trac.sagemath.org/ticket/29507