Disallow in tree CMake builds.
Disallow in tree CMake builds.

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

Change-Id: I4525ba56d4d125e2e7de1c25ed8fcd56ff4cd33b

GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom