libvpx: Add support for Visual Studio 2019

Apply recently added upstream Visual Studio 2019 support. Fixes #200 (closed)

(with edits by Nirbheek)

Signed-off-by: Nirbheek Chauhan nirbheek@centricular.com

CC: @seungha.yang

I found a bug in the backport which was causing the assembly files to not be built, which in turn was causing -good to not detect any features in the library because of undefined symbols.

The fix for master will be to bump libvpx instead, which is: !351 (merged)