Disable the -Wno-unused-const-variable Clang warning on Mac
Disable the -Wno-unused-const-variable Clang warning on Mac

In r4966 this was disabled on Linux for WebRTC code in order to detect any new unused const variables.
This CL does the same for Mac.

BUG=none
TEST=added an unused const and verified compilation fails
when this patch is applied. Mac trybots passing as well.

R=andrew@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/2459004

git-svn-id: 917f5d3ca488f358c4d40eaec14422cf392ccec9@4997 4adac7df-926f-26a2-2b94-8c16560cd09d

TimothyGu/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,mwgoldsmith/libilbc,mwgoldsmith/libilbc,ShiftMediaProject/libilbc,mwgoldsmith/ilbc,mwgoldsmith/ilbc,mwgoldsmith/libilbc,mwgoldsmith/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,mwgoldsmith/ilbc,ShiftMediaProject/libilbc,mwgoldsmith/ilbc