(In reply to Daniel Stone from comment #4)
> _BSD_SOURCE gets defined if you call AC_USE_SYSTEM_EXTENSIONS

Are you sure about this ? Looking at my autoconf 2.69
(/usr/share/autoconf/autoconf/specific.m4) and it sets the following

__EXTENSIONS__
_ALL_SOURCE
_GNU_SOURCE
_POSIX_PTHREAD_SEMANTICS
_TANDEM_SOURCE

Had no idea about the different interpretation of the defines between GNU vs
BSD libc(s). Thanks for that.

We can at check if the explicit define fixes things on Linux and ping the BSD
people for more robust solution ?