We get the following warning on build:

In file included from /usr/include/assert.h:35:
/usr/include/features.h:148:3: warning: "_BSD_SOURCE and _SVID_SOURCE are
      deprecated, use _DEFAULT_SOURCE" [-W#warnings]

This is a FAQ in the linux-man project [*]

  You are compiling the code on a system that has glibc 2.20 or later
  installed. In glibc 2.20, the _BSD_SOURCE and _SVID_SOURCE feature
  test macros were deprecated

[*] http://man7.org/tlpi/code/faq.html