Improve cmake build.  Add unittests to cmake build and automatically detect jpeg support. This change was originally generated to support the build of libyuv in naclports: https://chromium.googlesource.com/external/naclports/+/master/ports/libyuv/.  Also add cmake artifacts to .gitignore file.
BUG=366
TESTED=build and run unittests with cmake
R=harryjin@google.com

Review URL: https://webrtc-codereview.appspot.com/27009004

git-svn-id: http://libyuv.googlecode.com/svn/trunk@1146 16f28f9a-4ce2-e073-06de-1de4eb20be90