Support top-level make -jN
homology, chain complex fixes
Merge branch 'develop' into ticket/15624-parallel-make

Conflicts:
	Makefile
Trac #15730: small problems with CHomP, followup to #15161

This is a followup to #15161, fixing some problems which arose from the
changes there. In particular, if CHomP was installed, then the changes
at #15161 led to some doctest failures. These have mostly been
corrected, except for failures as in #12607 and also that CHomP seems to
be broken on my machine, so it is not computing homology groups
correctly. I think we should put off actually fixing CHomP to #15202.

URL: http://trac.sagemath.org/15730
Reported by: jhpalmieri
Ticket author(s): John Palmieri
Reviewer(s): Travis Scrimshaw
Trac #15624: Support "make -jN"

This should be supported by the top-level `Makefile`:
{{{
$ make -j4
}}}

The branch on this ticket implements this for sub-`make`s. For uses of
`$MAKE` outside of `make` itself (like building the Sage documentation
or Sage library), the value of `-j` given to `make` cannot be determined
so this cannot work.

URL: http://trac.sagemath.org/15624
Reported by: jdemeyer
Ticket author(s): Jeroen Demeyer
Reviewer(s): Peter Bruin