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 ahead of the predef headers. The solution was to: use the OSX
machine/endian.h header, and only allow a single OS to be detected at
once. Test cases added for both conditions on OSX.
backward PP compatability. Add BOOST_category_item_AVAILABLE as a
defined, or not, symbol for use with "#ifdef" or "#ifndef". Add
documentation of macros used for detection and version information. Also
replace direct use of BVN(0,0,0) and BVN(0,0,1) with more meaningful
macros that are globally defined, and hence consistent.
