libsubcmd: Make _FORTIFY_SOURCE defines dependent on the feature

Unconditionally defining _FORTIFY_SOURCE can break tools that don't work
with it, such as memory sanitizers:

  https://github.com/google/sanitizers/wiki/AddressSanitizer#faq

Fixes: 4b6ab94e

 ("perf subcmd: Create subcmd library")
Signed-off-by: Ian Rogers <irogers@google.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lore.kernel.org/lkml/20190925195924.152834-1-irogers@google.com


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