(split out from #30845)

git grep SAGE_LOCAL src/sage shows that there are still a few leftover places in the Sage library that hardcode SAGE_LOCAL as the directory where some executables etc. are found, notably gap.

In some settings, this forces us to set SAGE_LOCAL to a system location when ideally it should ideally be unset (as in distribution packaging). In particular, in the (experimental) set up of Conda for Sage developers described in https://wiki.sagemath.org/Conda, we would set SAGE_LOCAL to CONDA_PREFIX.

We adjust the configure script for this situation.

CC: @isuruf @tobiasdiez @kiwifb @orlitzky @dimpase

Component: build: configure

Author: Matthias Koeppe

Branch/Commit: ce6f8fd

Reviewer: Dima Pasechnik

Issue created by migration from https://trac.sagemath.org/ticket/33361