add gyp define to enable svn version check.  defaults to off so tests built outside of svn will build/run with a warning but not error.
BUG=356
TESTED=set GYP_DEFINES=target_arch=ia32 libyuv_disable_jpeg=1 libyuv_enable_svn=1
R=thorcarpenter@google.com

Review URL: https://webrtc-codereview.appspot.com/17329004

git-svn-id: http://libyuv.googlecode.com/svn/trunk@1075 16f28f9a-4ce2-e073-06de-1de4eb20be90