_BSD_SOURCE is deprecated since glibc 2.20

_BSD_SOURCE is deprecated since glibc 2.20
and will output a lot of warnings when compiling with
a new version of glibc.

Following the suggestions on
https://sourceware.org/glibc/wiki/Release/2.20#Packaging_Changes

The first step is to try to remove the define _BSD_SOURCE
(in case it's not really needed)
If that's not possible you can define both _BSD_SOURCE and
_DEFAULT_SOURCE, so you are both backward and
forward compatible.