Improve documentation of projective dynatomic polynomial
Updated the docstring of LinearMatroid.extensions() and .coextensions(). It is now explicitly stated that
only (co)rank-preserving (co)extensions are generated, and not the extension by a coloop, nor the coextension
by a loop.
Use autotools for csage
Trac #15794: Matroid linear_extensions() does not mention lack of coloops.

The linear_extensions() docstring does not mention that the extension by
a coloop is never computed.

URL: http://trac.sagemath.org/15794
Reported by: yomcat
Ticket author(s): Rudi Pendavingh
Reviewer(s): Michael Welsh
Trac #15490: improve documentation of projective dynatomic polynomial

There are two current issues. There is a typo: `n` instead of `period`.

Also, it would be nice to verify that the roots are actually giving
preperiodic points of the correct period as a doctest.

URL: http://trac.sagemath.org/15490
Reported by: bhutz
Ticket author(s): Weixin Wu
Reviewer(s): Ben Hutz
Trac #15594: Use autotools for csage

Use autotools instead of scons to build `c_lib`. After this change,
polybori is the only part of Sage using scons.

URL: http://trac.sagemath.org/15594
Reported by: ohanar
Ticket author(s): R. Andrew Ohana, Jeroen Demeyer
Reviewer(s): R. Andrew Ohana, Jeroen Demeyer
Updated Sage version to 6.2.beta3