Add build.head for building gstreamer from master

When I tried to build gstreamer from master git, several features were required.
These patch set contain results of my trials and errors to build gstreamer matters.

1. Switching from url to mirror explicitly when an official site is down. 
2. Upgrading gnutls, nettle. Actually, I tried to add mirror for them, but I do more.
3. Adding build.head for gstreamer matters and fixing dependencies.