Dependencies, e.g. PPL for GLPK

On macOS with ppl installed via Homebrew,
the configure script reported the following:

## ---------------------------------------------------- ##
## Checking whether SageMath should install SPKG ppl... ##
## ---------------------------------------------------- ##
configure:27878: checking whether any of glpk gmp mpir
is installed as or will be installed as SPKG
configure:27882: result: yes; install ppl as well
configure:28277: no suitable system package found for SPKG ppl

and eventually recommended to brew install ppl.

It would seem that, in the above, this is correct:

configure:27878: checking whether any of glpk gmp mpir
is installed as or will be installed as SPKG
configure:27882: result: yes; install ppl as well

but this does not really follow:

configure:28277: no suitable system package found for SPKG ppl

In this case, because at least one of (glpk, gmp, mpir)
either was or would be installed as an SPKG, the script
concluded ppl should be installed as an SPKG too.

So the decision to install ppl was based on that,
not on the absence of ppl. In such a case, maybe
the script should not recommend installing ppl
(at least not without checking if it is actually there).

Initial report:
sage-release.

Python vs Python3, Python 3.7 vs Python 3.8

On macOS 10.14.6, as of July 2020, Homebrew installs Python 3.8.

Still, configure, looking for Python 3.7, recommends to brew install python3.

See also

• Meta-ticket: Refactor and improve system package related scripts, tox.ini, build/bin/write_dockerfile.sh #29146: Meta-ticket: Refactor and improve system package related scripts, tox.ini, build/bin/write_dockerfile.sh

• refine definition of variable SAGE_NEED_SYSTEM_PACKAGES in m4/sage_spkg_collect.m4 #30863: refine definition of variable SAGE_NEED_SYSTEM_PACKAGES in m4/sage_spkg_collect.m4

CC: @mkoeppe @slel @mwageringel @dimpase @EmmanuelCharpentier

Component: build: configure

Issue created by migration from https://trac.sagemath.org/ticket/29586