Manage depot_tools as an external project

[aisouard]

First step for #30

• Retrieve the depot_tools repository with CMake
• Set the environment variable inside prefix.sh to disable auto-update
• Use gclient to retrieve WebRTC with the desired commit / release branch head
• Checkout depot_tools to a commit having the closest date with WebRTC's

[aisouard]

Refactored a lot of code and files in b59291f.
depot_tools is managed until we set a DEPOT_TOOLS_PATH variable.
WebRTC is manually fetched and sync'd with the right depot_tools revision.
I was finally able to remove libwebrtc_add_command macros, the code is cleaner and more elegant.

[aisouard]

Fixed in the refactor-depot-tools branch