integrate sagelib build systems into old toplevel

- build/install
  - export SAGE_SCRIPTS_DIR
    .. required to override some magic happening later
  - sagedist/bin to path in install
    .. that's where sage-spkg is
- build/deps
  - configure src/{bin,doc}
  - use stamps
python-sage build system completion/cleanup

- LIBCSAGE_INCLUDES
- FACTORY_INCLUDES
- SINGULAR_INCLUDES
python-sage: env

- also expand braced variables
  .. so they work in sh *and* make *and* env.py
- rearrange LIBCSAGE_INCLUDES
sage-bin env
sagelib bsc
patch libgap
python-sage: env
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