Add _FORTIFY_SOURCE support for snprintf, vsnprintf
At this time, we opt out of these protections for clang, as clang
does not implement __builtin_va_arg_pack().
http://clang.llvm.org/docs/UsersManual.html#c_unimpl_gcc
Change-Id: I73ebe5ec8dad1dca8898a76d6afb693a25f75375
