Most changes from review comments implemented. Remove use of "#elif" for

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.

include/boost/predef/os/linux.h

@@ -24,11 +24,15 @@ http://www.boost.org/LICENSE_1_0.txt)
     ]
  */
 
-#define BOOST_OS_LINUX BOOST_VERSION_NUMBER(0,0,0)
+#define BOOST_OS_LINUX BOOST_VERSION_NUMBER_NOT_AVAILABLE
 
 #if defined(linux) || defined(__linux)
 #   undef BOOST_OS_LINUX
-#   define BOOST_OS_LINUX BOOST_VERSION_NUMBER(0,0,1)
+#   define BOOST_OS_LINUX BOOST_VERSION_NUMBER_AVAILABLE
+#endif
+
+#if BOOST_OS_LINUX
+#   define BOOST_OS_LINUX_AVAILABLE
 #endif
 
 #define BOOST_OS_LINUX_NAME "Linux"