Some releases of g++, on some platforms, whilst running under some
standards, may not define neither linux, nor __linux. Add detections
for __linux__ and __gnu_linux__ for robustness.
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.
