Fix redef of OS_BSD predef.

When a BSD sub-os header is included directly the general
OS_BSD predef is inidially defined as not-available. Which
warns as the sub-os header defines it. The fix is to unconditionally
un-def the OS_BSD predef.

fixes https://github.com/boostorg/predef/issues/111

include/boost/predef/os/bsd/bsdi.h

@@ -29,6 +29,7 @@ http://en.wikipedia.org/wiki/BSD/OS[BSDi BSD/OS] operating system.
     defined(__bsdi__) \
     )
 #   ifndef BOOST_OS_BSD_AVAILABLE
+#       undef BOOST_OS_BSD
 #       define BOOST_OS_BSD BOOST_VERSION_NUMBER_AVAILABLE
 #       define BOOST_OS_BSD_AVAILABLE
 #   endif

include/boost/predef/os/bsd/dragonfly.h

@@ -29,6 +29,7 @@ http://en.wikipedia.org/wiki/DragonFly_BSD[DragonFly BSD] operating system.
     defined(__DragonFly__) \
     )
 #   ifndef BOOST_OS_BSD_AVAILABLE
+#       undef BOOST_OS_BSD
 #       define BOOST_OS_BSD BOOST_VERSION_NUMBER_AVAILABLE
 #       define BOOST_OS_BSD_AVAILABLE
 #   endif

include/boost/predef/os/bsd/free.h

@@ -31,6 +31,7 @@ http://en.wikipedia.org/wiki/Freebsd[FreeBSD] operating system.
     defined(__FreeBSD__) \
     )
 #   ifndef BOOST_OS_BSD_AVAILABLE
+#       undef BOOST_OS_BSD
 #       define BOOST_OS_BSD BOOST_VERSION_NUMBER_AVAILABLE
 #       define BOOST_OS_BSD_AVAILABLE
 #   endif

include/boost/predef/os/bsd/net.h

@@ -36,6 +36,7 @@ http://en.wikipedia.org/wiki/Netbsd[NetBSD] operating system.
     defined(__NETBSD__) || defined(__NetBSD__) \
     )
 #   ifndef BOOST_OS_BSD_AVAILABLE
+#       undef BOOST_OS_BSD
 #       define BOOST_OS_BSD BOOST_VERSION_NUMBER_AVAILABLE
 #       define BOOST_OS_BSD_AVAILABLE
 #   endif

include/boost/predef/os/bsd/open.h

@@ -80,6 +80,7 @@ http://en.wikipedia.org/wiki/Openbsd[OpenBSD] operating system.
     defined(__OpenBSD__) \
     )
 #   ifndef BOOST_OS_BSD_AVAILABLE
+#       undef BOOST_OS_BSD
 #       define BOOST_OS_BSD BOOST_VERSION_NUMBER_AVAILABLE
 #       define BOOST_OS_BSD_AVAILABLE
 #   endif