This one was only defined if _GNU_SOURCE was set in older glibc
versions, check that and provide the define in such cases.

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: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/n/tip-ilsgsysr6s3mru7rf2befnu5@git.kernel.orgSigned-off-by: NArnaldo Carvalho de Melo <acme@redhat.com>