"make download" target for sage

[vbraun]

Moved here from the github page: robertwb/sage@ff45683

Component: misc

Keywords: sd53

Author: Robert Bradshaw

Branch/Commit: u/vbraun/make_download @ 383c35a

Reviewer: Volker Braun

Issue created by migration from https://trac.sagemath.org/ticket/15222

[sagetrac-git]

Branch pushed to git repo; I updated commit sha1. Last 10 new commits:

[changeset:383c35a]	Makefile target to download upstream sources.
[changeset:3b15578]	Merging Sage-5.12.beta5, newest dev scripts, and the doctest fixes.
[changeset:1456c52]	Merge branch 'ticket/14482' into public/sage-git/master
[changeset:b890215]	Merge branch 'ticket/14482' into public/sage-git/master
[changeset:d8713eb]	Merge remote-tracking branch 'origin/build_system' into public/sage-git/master
[changeset:970090d]	Merge branch 'u/ohanar/build_system'
[changeset:cf14c84]	Merge branch 'ticket/14482' into public/sage-git/master
[changeset:060184c]	Merge branch 'ticket/14482' into public/sage-git/master
[changeset:b95a820]	Merge branch 'ticket/14482' into public/sage-git/master
[changeset:a10bc4f]	Merge branch 'ticket/14482'

[sagetrac-git]

Commit: 383c35a

[vbraun]

comment:3

Looks good to me...