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/language/objc.h

@@ -23,11 +23,15 @@ http://www.boost.org/LICENSE_1_0.txt)
     ]
  */
 
-#define BOOST_LANG_OBJC BOOST_VERSION_NUMBER(0,0,0)
+#define BOOST_LANG_OBJC BOOST_VERSION_NUMBER_NOT_AVAILABLE
 
 #if defined(__OBJC__)
 #   undef BOOST_LANG_OBJC
-#   define BOOST_LANG_OBJC BOOST_VERSION_NUMBER(0,0,1)
+#   define BOOST_LANG_OBJC BOOST_VERSION_NUMBER_AVAILABLE
+#endif
+
+#if BOOST_LANG_OBJC
+#   define BOOST_LANG_OBJC_AVAILABLE
 #endif
 
 #define BOOST_LANG_OBJC_NAME "Objective-C"