disable _FORTIFY_SOURCE under clang

Clang and _FORTIFY_SOURCE are just plain incompatible with
each other.  First of all, clang doesn't understand the
__attribute__((gnu_inline)) header. Second of all,
Clang doesn't have support for __builtin_va_arg_pack()
and __builtin_va_arg_pack_len() (see
http://clang.llvm.org/docs/UsersManual.html#c_unimpl_gcc)

Until we can resolve these issues, don't even try using
_FORTIFY_SOURCE under clang.

Change-Id: I81c2b8073bb3276fa9a4a6b93c427b641038356a