toplevel: build system (#14796-2)

- GNUmakefile.am, configure.ac
- autogen.sh
- empty NEWS, AUTHORS, ChangeLog
- move README.txt to README
- find GNU tar
- core module build rules
- gcc check
- newest_version (from install.in)
- skip "prereq" stuff (for now?)
- change variable names to something more useful
- urlgrabber
- wrapper Makefile
- deps.in
- dist helpers (install, make)
- foreign package en/disable (m4)
- ...
toplevel src versioning (experimental)
toplevel simplify package source version (#14792)

just create symlink <pkgdir>src->actual_package_source-version
toplevel: keepbuild option, WIP, see #14792
toplevel build system continued

- package-(un)install
- TODO
- dist environment
- sage-dist-install program
- sage-setup.py wrapper
toplevel: gitignore
cython: add -MF switch to write dependency files (#14728)

... and -M -F -D -P
sagelib: build system (#14807)

this implements a plain ./configure && make interface for the parts
located in /src
sage-ext: build-system
sage-doc: move files
sage-doc: build system
mpir: quick hack

- use $CUR, not ..
- just complain about patches
- FIXME: rework
singular: spkg-install (makefile)
singular: misc patches

- dont -I$(prefix), its pretty much nonsense
- -I../factory, as there might be headers...
- dont recreate cf_gmp.h
- dont unnecessarily touch mod2.h
sage-bin: move files (make templates)
sage-bin: sage-build
sage-bin: sage-spkg.in
sage-bin: sage.in

- config files to SAGE_ETC=@sysconfdir@
sage-bin build system

- add environment for build
- SAGE64 maybe incomplete
sage-bin: simplify sage-sdist