pkgconfig package, incomplete
toplevel: move files (#14796-1)

(extra commit, to help git)
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
- decruft-target and package-distcheck
- sort out compiler versions (thanks, Julian)
- simplify deps.in
toplevel bs sq
toplevel: .gitignore
toplevel: some more deps
toplevel build system continued
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

- SAGE_DOC_SRC
- quick hacks
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@