csage: gmp or mpir switch
Merge autotools spkg into sagelib

this will be required to build sagelib from git.
build autotools before csage

(unconditionally, will be fixed in #14796)
Merge branch 'csage_build_system' into sagelib
move pkgconfig into the right spot
move src/bin/sage-spkg.in -> sagedist/bin/sage-spkg (#14807)

src/bin might not be accessible before sagelib has been
unpacked/configure/staged/installed. the old toplevel build system
requires sage-spkg earlier.
provide sage-env for sagedist. temporary workaround for sagelib (#14807)
Merge branch 'sagelib' into toplevel

Conflicts:
	build/deps (deleted)
python-sage: whitelist python2.7 multiprocessing in stackframes test
python-sage: comment on header hack
python-sage: make libgap obsolete (I)
python-sage: make libgap obsolete, II (patches against gap upstream)
cython: add -MF switch to write dependency files (#14728)

... and -M -F -D -P
cython: patch: gdb output dir option
sage-doc: build system
sage-doc. use SAGE_DOC_SRC as doc source path

this is required if srcdir!=builddir. otherwise it doesn't make a
difference
sage-bin: env

- adapt env
- remove sage-arch-env
sage-bin: remove ECLDIR from env.sh (ecl takes care of it)
sage-bin: sage-build
sage-bin: sage-spkg.in