* objc/objc.h (__GNU_LIBOBJC__): New #define providing an easy way
        to check the API version.  Added some comments.

        * objc-private/common.h: New file.
        * NXConstStr.m: Include objc-private/common.h.
        * Object.m: Same change.
        * Protocol.m: Same change.
        * archive.c: Same change.
        * class.c: Same change.
        * encoding.c: Same change.
        * exception.c: Same change.
        * gc.c: Same change.
        * hash.c: Same change.
        * init.c: Same change.
        * libobjc_entry.c: Same change.
        * linking.m: Same change.
        * misc.c: Same change (and added a comment).
        * nil_method.c: Same change.
        * objects.c: Same change.
        * sarray.c: Same change.
        * selector.c: Same change.
        * sendmsg.c: Same change.
        * thr.c: Same change.

From-SVN: r164218