toplevel build system update

 - decruft target and package-distcheck
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
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
- use install, not cp
- chmod *before* install
python: patch
python: (experimental) set rpath in python executable
python: use sage-dist-install
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
- remove sage-arch-env
sage-bin: simplify sage-sdist
foreign: convert spkg-install to makefiles

- pexpect
- rpy2
foreign: spkg_install hacks/wip

- atlas: patch workaround
- cliquer: FIXME: patch build system to set -fno-ivopts accordingly
- cliquer: patches, mkdir
- cliquer: remove testcc.sh call, pulls in too much cruft
- conway_polynomials minor fixes
- conway_polynomials/spkg-install
- cvxopt patch
- flintqs/spkg-install
- gap: patches, do not force gmp location (might be wrong)
- gdmodule
- gfan patch
- iconv: need fix
- lapack/spkg-install
- lcalc: patch workaround
- matplotlib
- maxima patch
- mpfr minor
- mpfr/spkg-install
- mpir: just complain about applied/failing patches, files
- mpir/spkg-install
- ntl: hack
- pari: make gprc.expect accessible
- pari: patch workaround
- pari/spkg-install
- pil complain about patch
- polybori: make custom.py accessible
- polybori: patch workaround
- pygments: just report patch error
- python: allow patches already applied
- python: patch workaround
- ratpoints spkg-install
- readline/spkg-install
- sphinx: just complain about patches
- symmetrica: default values in makefile, patch workaround
- termcap/spkg-install
atlas: trying to make it work