glibc devs have apparently decided _BSD_SOURCE will be
deprecated on Linux, and issue a preprocessor warning if
declaring _BSD_SOURCE without _DEFAULT_SOURCE.

https://sourceware.org/glibc/wiki/Release/2.20

Fixes #8397.

LGTM=iant
R=dave, gobot, iant
CC=golang-codereviews
https://golang.org/cl/112530043