Prepare to stop using -std=gnu*

This CL is necessary for:
https://chromium-review.googlesource.com/c/chromium/src/+/1149337

Some third party changes are necessary, so code of the form
  #ifndef _GNU_SOURCE
  #define _GNU_SOURCE
  #endif
will be necessary in the meantime.

BUG=427584

Change-Id: I89b3640d35da5fd8666c69d21836212558140c00
Reviewed-on: https://chromium-review.googlesource.com/1149336
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#578046}
5 files changed