pigz: update to more stable download URL

pigz: update to more stable download URL

The pigz website seems to only host the latest release tarball.
Chances are that the 2.4 tarball will disappear when 2.5 comes out.

Instead, use the Github tarballs provided by the developer. Comparison
between the 2.4 / v2.4 tarballs shows that only the .gitignore file is
added.

Suggested-by: Alfredo Alvarez Fernandez <alfredo.alvarez_fernandez@nokia.com>
Signed-off-by: Thomas De Schampheleire <thomas.de_schampheleire@nokia.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>