How to enable BSD defined names / functions

There's rather a lot of software that defines _POSIX_C_SOURCE to some value
(usually 200809L for POSIX-1.2008), but also wants to use other things.

One can generally work around this issue by defining __BSD_VISIBLE=1, but
most other systems have something more specific. NetBSD has _NETBSD_SOURCE.
OpenBSD has _OPENBSD_SOURCE as well. There's also some expectation that
_BSD_SOURCE can be defined, but none of the currently active BSDs has that.

It appears from casual inspection that _NETBSD_SOURCE=1 means approximately
the same as __BSD_VISIBLE=1 in FreeBSD.

Would it make sense to add a _FREEBSD_SOURCE=1 case and have it include
__BSD_VISIBLE=1 regardless  of what other macros (especially
_POSIX_C_SOURCE) are defined to be a more regimented and defined way to
expand the namespace when multiple namespaces are defined?

Warner