sagenb: use SAGE_DOC_SRC to locate doc source (#15101)
Merge branches 'cython_dbg' and 'cython_deps' into python_sage_build

the python library is more fun with dependencies and debugging info
Merge branches 'sage_doc', 'python_sage_build' and 'csage_build_system' into sagelib

Conflicts:
	build/deps (autotools rule)
python-sage fix two references to libsagegap
unhardwire ELLCURVES_DATA_DIR
qepcard doctest fix: singular might well be in /usr/bin, not .../local/bin
boehm_gc: spkg-install
cddlib: fix permissions
cephes: patch quickfix
cliquer: spkg-install

- idempotence
- use sage-dist-install
conway_polynomials: spkg-install

environment would be not necessary, if conway_polynomials was built
before sagelib (see debian package)
cvxopt: spkg-install, idempotence, filelist
cython: spkg-install

- turn into makefile (looks simpler, is simpler)
- repair env
- use sag-dist-install
docutils: use sage-setup.py
ecl: spkg-install

- unhardcode gmp includedir
- patching
- FILELIST
- environment
- spkg-check wrapper
eclib: spkg

- use sage-dist-install
- unhardcode paths
ecm: spkg-install misc

- patch idempotence
- check target
- unhardwire gmp etc.
elliptic_curves: spkg-install, fix idempotence
fflas_ffpack: spkg-install, fix idempotence
flint: spkg-install

- idempotence
- use sage-dist-install