If not, tell the user that:

  config/Makefile:273: Old libdw.h, finding variables at given 'perf probe' point will not work, install elfutils-devel/libdw-dev >= 0.157

And return -ENOTSUPP in die_get_var_range(), failing features that
need it, like the one pointed out above.

This fixes the build on older systems, such as Ubuntu 12.04.5.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Vinson Lee <vlee@freedesktop.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/n/tip-9l7luqkq4gfnx7vrklkq4obs@git.kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>