Might close #4907 and #4904 (tbc)

wget uses the timestamp from the server, which is nice in theory, but
breaks makefile dependency propagation.

The automatic packages selection can be overriden.

And DxVA2 (why is that in DirectShow?!).
And Direct2D (untested - does not work in older contrib for me).

 * support for fetching only needed sources
 * always building from source tarball:
   - easier GPL compliance
   - easier offline/firewalled builds
 * shared source tarballs for all targets
 * dynamic package selection, no static distribution files
 * fixed file dependency propagation with delete on errors
 * fixed directory dependencies using atomic rename

Only ogg and tremor are in this initial version. This focuses on the
core of the system. The bulk of packages still need to be ported.