This ticket adds an spkg-configure.m4 for cvxopt and various other python packages, to allow them to be used from the system (part of #27330).

These python packages are special because we currently use a python "venv" that is isolated from the system. Thus this ticket proceeds in several steps:

1. Add an --enable-system-site-packages flag to the top-level ./configure script. When present, this will tell sage-venv that the venv it creates should not be isolated from the system.
2. Create a new macro that can detect python packages on the system. It should ensure that the version bounds in each python package's install-requires.txt are met.
3. Create the spkg-configure.m4 files themselves, which should be relatively simple at that point.
4. Test everything thoroughly with various combinations of system and non-system python packages.

Platform-specific testing:

• for example TARGETS_PRE="build-local" EXTRA_CONFIGURE_ARGS="--enable-system-site-packages" tox -e docker-ubuntu-focal-standard -- build ptest

Depends on #32703
Depends on #33067

Component: build: configure

Keywords: cvxopt, system packages, Python library

Author: Michael Orlitzky

Branch/Commit: u/mjo/ticket/29665 @ baf6301

Issue created by migration from https://trac.sagemath.org/ticket/29665