Makes the build work with CGO_CPPFLAGS=-D_GNU_SOURCE,
as reportedly used by TinyGo.
Fixes #66325
Change-Id: I794f1cd89814638fdb6c3066d13bbd7da88c9d93
Reviewed-on: https://go-review.googlesource.com/c/go/+/571875
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Ian Lance Taylor <iant@golang.org>
Commit-Queue: Ian Lance Taylor <iant@golang.org> Reviewed-by: Damien Neil <dneil@google.com> Reviewed-by: Cherry Mui <cherryyz@google.com>