perf tools: Pick up libdw without explicit LIBDW_DIR

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 <[email protected]>
Acked-by: Jean Pihet <[email protected]>
Cc: Arnaldo Carvalho de Melo <[email protected]>
Cc: Jiri Olsa <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Jiri Olsa <[email protected]>