add support for specifying alternative download methods

[boegel]

(old internal ticket 317)

We need to be able to also specify alternative ways of downloading sources, e.g. checking out from a CVS/SVN/git/hg repository.

This is required for e.g. MTL4 (see issue #111), CP2K, ...

[boegel]

This was requested by a number of people during SC'12, so let's bump the priority a bit (low --> medium).

[gribozavr]

Please don't forget about hg repositories.

[gribozavr]

My usecase for this feature is Clang. There are revisions in the repository that received more testing, but are not released (and will never be packaged as tarballs). Clang development moves quite fast, and the latest release -- 3.2 can be considered somewhat dated now. So it would make sense to build from these revisions while fetching the source directly from VCS.

[boegel]

For SVN, we should make sure support for downloading specific svn revisions is added, e.g. to download the source code for OpenSees v2.4.3:

svn co -r 5621 svn://opensees.berkeley.edu/usr/local/svn/OpenSees/trunk OpenSees

see http://opensees.berkeley.edu/OpenSees/developer/svn.php (http://opensees.berkeley.edu/OpenSees/changeLog.php for the SVN revision that corresponds to a particular version)

[JensTimmerman]

I've started on adding support for downloading over svn, see
#1082

The path chosen here is: export a revision of the svn repo (like checkout but without revision information), create a tarball, put this tarball in the sources folder, just like a normal http download would do.

This is already working for me, just cleaning up the code a bit now.

[akesandgren]

Cf, #2555

[boegel]

I'll go ahead and close this, since we have support for cloning a git repo.

Also supporting other methods like SVN can be done equivalently.