the external install should have glpsol and libglpk

On debian they are in glpk-utils and libglpk-dev packages.

4ti2 needs to know its prefix (or probably just --with-glpk should be there).
Anyhow, we introduce a new env. var. SAGE_GLPK_PREFIX
which is '' for system's glpk and SAGE_LOCAL otherwise.

cvxopt for its glpk extension needs to know the (non-standard) include directory and
library directory of glpk, so they can be made blank
for the stanard locations --- but as in particular libPython comes from SAGE_LOCAL, this should be left unchanged for the time being.

Sage's glpk needs a specific patch which is not upstreamed, so we need to work this out, too.

Depends on #24824

CC: @embray @kiwifb @isuruf @antonio-rojas @mkoeppe

Component: build: configure

Author: Dima Pasechnik

Branch/Commit: e6bb511

Reviewer: Isuru Fernando

Issue created by migration from https://trac.sagemath.org/ticket/28459