This is a follow-up on #28788, which implemented the current version of the "Build status for each package" displayed at the end of configure.

For an optional package that is not installed already, it displays a message like the following:

e_antic-0.1.5: does not support check for system package; optional, use "./configure --enable-e_antic" to install

In this ticket, we change the message if the package has been configured to be installed.

To this end, we introduce the variable SAGE_ENABLE_<SPKG> for each optional/experimental SPKG. We also AC_SUBST these variables for future use by the build system.

CC: @dimpase @jhpalmieri @orlitzky @slel

Component: build: configure

Author: Matthias Koeppe

Branch/Commit: 22d7ae4

Reviewer: Dima Pasechnik

Issue created by migration from https://trac.sagemath.org/ticket/29363