It is hard to control what flags consumers may try to build us with. Account for someone adding _GNU_SOURCE to the build line. Change-Id: I4c931da70a9dccc89382ce9100c228c29d28d4bf Reviewed-on: https://boringssl-review.googlesource.com/13621 Commit-Queue: David Benjamin <davidben@google.com> Commit-Queue: Adam Langley <agl@google.com> Reviewed-by: Adam Langley <agl@google.com> CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>