Fix "_FORTIFY_SOURCE" redefined warning

On Ubuntu 12.04, we got the following warning message:
<command-line>:0:0: warning: "_FORTIFY_SOURCE" redefined
 [enabled by default]
<built-in>:0:0: note: this is the location of the previous definition

This was already fixed in VP8 configure file. Did the same change in
experimental branch to stop this warning.

Change-Id: Id162e5fd8841585ae806df6560b2f7536ea307c0