Makefile: try next download url on failed checksum

There are various cases where a download succeeds but the checksum
is wrong:
  - upstream re-releases tarball without new version
  - sourceforge returns 200 with a text message about outage
  - other strange glitches

In these cases, we currently fail and stop the build. This PR allows
for falling through the next URLs until it will eventually get to
the checksum-suffixed version.

This ensures builds are always (potentially) repeatable as the checksum
associated with the version is reachable. Next step would be to change
all the backups to use suffixed tarballs.

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