cmake: Include relative path to source dir in config string.
cmake: Include relative path to source dir in config string.

With this addition it's possible to copy/paste the string into
a console for use in running cmake directly.

Change-Id: I9f920337f6470e221eed17f925885811c76ddb9e

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