Clean up SAGE_ROOT directory by moving stuff into SAGE_ROOT/build/

[mkoeppe]

• Move configure.ac, m4, config, Makefile into build/.
• Leave behind symlinks configure -> build/configure, Makefile -> build/Makefile (or -> build/make/Makefile-main)
• Split out build/bootstrap from SAGE_ROOT/bootstrap, call it from there.

CC: @dimpase @orlitzky @jhpalmieri

Component: build

Issue created by migration from https://trac.sagemath.org/ticket/30025

[mkoeppe]

superseded by #31662