cmake: Add partial configure.

- Add minimal compiler flag testing.
- Generate aom_config.c and aom_config.h. Note: hard coded
  to generic-gnu values for now.
- Still a work in progress. This will not build anything.

BUG=https://bugs.chromium.org/p/aomedia/issues/detail?id=76

Change-Id: Id65b42ea9f4c4f744d788660e2de7234886ce039
5 files changed