cython: add -MF switch to write dependency files (#14728)

... and -M -F -D -P
add more autotools executables

(I can hack, too. next time, read the manual)
cython: patch: gdb output dir option
Merge branch 'autotools' into csage_build_system
build autotools before csage

(unconditionally, will be fixed in #14796)
csage: gmp_or_mpir fixup
provide gmpxx_or_mpirxx.h (required by sage-python)
Merge branch 'gmp_mpir' into csage_build_system
unhardwire polymake path
unhardwire sandpile
python-sage: whitelist python2.7 multiprocessing in stackframes test
sage-doc: build system. mostly dist
sage-doc. use SAGE_DOC_SRC as doc source path

this is required if srcdir!=builddir. otherwise it doesn't make a
difference
make flint work with gmp

this is only active in case USE_GMP is set.
sagelib: environment for check target
sage-bin env
python-sage build system

- fix LIBCSAGE_INCLUDES
- TODO
- remove some stat checks
- expand some tabs in module_list
- my_LD_LIBRARY_PATH overridable
python-sage: env

- also expand braced variables
  .. so they work in sh *and* make *and* env.py
- rearrange LIBCSAGE_INCLUDES
- split includes into lists
- fix SAGE_SRC
python-sage doctest path tolerance
Merge branches 'cython_dbg' and 'cython_deps' into python_sage_build