The wget download apparently uses the time stamp of the file on the
server by default, which breaks incremental builds.

If wget downloads the file and it does not get the current timestamp at
the time of download but the one the file happens to have on the server,
then if you built from the previous version of the tarball at a later
time than the timestamp of the new tarball, make won't detect that it
has to unpack the new tarball.

The curl download should not be affected since curl requires an explicit
-R to use the server side time stamp.

Change-Id: I6bab51d20b8ab5e485fe68d3f27c31aaddc99f68