constructormagic.h macros are duplicated in several repositories. undef them in webrtc to prevent conflict for some build configurations.

BUG=N/A
R=minyue@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/18619004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6554 4adac7df-926f-26a2-2b94-8c16560cd09d