cmake support: A starting point.
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

GrokImageCompression/aom,mbebenita/aom,smarter/aom,GrokImageCompression/aom,mbebenita/aom,smarter/aom,mbebenita/aom,smarter/aom,luctrudeau/aom,luctrudeau/aom,GrokImageCompression/aom,mbebenita/aom,mbebenita/aom,luctrudeau/aom,luctrudeau/aom,smarter/aom,luctrudeau/aom,mbebenita/aom,smarter/aom,GrokImageCompression/aom,mbebenita/aom,mbebenita/aom,smarter/aom,mbebenita/aom,GrokImageCompression/aom,luctrudeau/aom,GrokImageCompression/aom