This ticket changes the build process of sagelib in the following way:

• src/Makefile delegates ALL building to src/setup.py
• src/setup.py no longer depends on environment variables $SAGE_ROOT, $SAGE_SRC, $SAGE_DOC_SRC etc. (to demonstrate this, Makefile poisons these environment variables). It still depends on $SAGE_LOCAL and environment variables that point below it.

This ticket is meant as:

• preparation for VPATH builds of sage-the-distribution (Enable VPATH builds (several independent build trees connected to one source tree) #21469)
• working towards the goal of making sagelib pip-installable -- see Meta-ticket: Make sagelib a pip-installable Python source package, listed on PyPI #21507 for the eventual goal of having sagelib on PyPI
• making the flow of directory information at build time clearer for developers

More specifically, the goal of this ticket is that only SAGE_LOCAL needs to be set when the user does 'pip install' of the sagelib. (This ticket almost achieves this, except it also needs SAGE_PKGS and SAGE_CYTHONIZED to be set. The hope is that #20382 and other future tickets will develop better mechanisms to communicate package and directory information to the build.)

. . . . . . .

Some possibly useful information:

• Documentation on distutils (https://docs.python.org/2/install/), describing use of --build-base to do VPATH builds.
• pip install keeps the source directory clean, building instead in a temporary directory, by copying the sources.
  pip install also offers options --build to select a build directory, but there are some pip issues: 2060, 2053, 804 that affect this
• Transition to build system for sage (the library) #14807 has some tricks to making VPATH builds work without copying all python source files. But it uses automake instead of setup.sh; we will not do this in our ticket.

configure tarball: http://sage.ugent.be/www/jdemeyer/sage/configure-185.tar.gz

CC: @sagetrac-felixs @jdemeyer @kiwifb @embray @nexttime @vbraun @dimpase @jhpalmieri @videlec @saraedum @seblabbe @nthiery @mezzarobba

Component: build

Author: Matthias Koeppe

Branch/Commit: 0c2ac95

Reviewer: Jeroen Demeyer

Issue created by migration from https://trac.sagemath.org/ticket/21480