cmake support: A starting point.
Start adding cmake build support. This is based on the generic-gnu target and will not build anything. It simply produces a project file (when generating for a IDE) that can be loaded and that allows for interaction with (most of) the aom sources used in a generic-gnu build. Notable missing pieces: - flag testing - config generation - experiment configuration - enable/disable encoder/decoder - aomenc/aomdec - all third party library build integration - all tests Change-Id: Iaeda0b03d58591a26a8fb54f63a2aa3b5354e3a6