tools lib api: Undefine _FORTIFY_SOURCE before setting it

Some toolchains (like Hardened Gentoo) define _FORTIFY_SOURCE in the
built-in, default args.  This causes perf builds to fail with:

<command-line>:0:0: error: "_FORTIFY_SOURCE" redefined [-Werror]
<built-in>: note: this is the location of the previous definition cc1:
all warnings being treated as errors

To avoid this, undefine _FORTIFY_SOURCE before (possibly re-)defining it
in tools/lib/api.

v2 applies cleanly on top of already pulled kbuild changes for 4.1-rc1.

Signed-off-by: Bobby Powers <bobbypowers@gmail.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Cc: Dirk Gouders <dirk@gouders.net>
Cc: Michal Marek <mmarek@suse.cz>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: linux-kbuild@vger.kernel.org
Link: http://lkml.kernel.org/r/1429658381-3039-1-git-send-email-bobbypowers@gmail.com


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>