bsd-variables
-------------

* The :variable:`BSD` variable is set to true when the target system
  is BSD (DragonFlyBSD, FreeBSD, OpenBSD, or NetBSD)
* The :variable:`DRAGONFLYBSD` variable is set to true when the target system is DragonFlyBSD.
* The :variable:`FREEBSD` variable is set to true when the target system is FreeBSD.
* The :variable:`NETBSD` variable is set to true when the target system is NetBSD.
* The :variable:`OPENBSD` variable is set to true when the target system is OpenBSD.
