perf tools: Pick up libdw without explicit LIBDW_DIR

The Makefile logic sets FEATURE_CHECKS_CFLAGS-libdw-dwarf-unwind and
FEATURE_CHECKS_LDFLAGS-libdw-dwarf-unwind only if LIBDW_DIR is
defined. This means that under a normal setup,

  $ make NO_LIBUNWIND=1

won't automatically pick up libdw. Fix this.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
Acked-by: Jean Pihet <jean.pihet@linaro.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Link: http://lkml.kernel.org/r/1395873845-466-1-git-send-email-artagnon@gmail.com
Signed-off-by: Jiri Olsa <jolsa@redhat.com>