Create symbolic links the <so-version> is specified.

* tools/stage.jam
  (symlink-lib): New rule.
  (stage-shared-linking-generator.run): Use the above to
  create symlinks.


[SVN r26415]

tools/build

@@ -1 +1 @@
-Subproject commit 6afa38681d89e72a2f701f42346a5ab0f551f1fa
+Subproject commit d2b38551f36272f0dac8ecf6fd6da1ccda6af4d2