link keyword for :dir argument of ld and include-book

Example Form:
(add-include-book-dir :smith "/u/smith/")
 ; For (include-book "foo" :dir :smith), prepend "/u/smith/" to "foo".

General Form:
(add-include-book-dir kwd dir)
where kwd is a keywordp and dir is the pathname of a directory. (If the final '/' is missing, ACL2 will add it for you.) The effect of this event is to modify the meaning of the :dir keyword argument of include-book as indicated by the examples above, and similarly for ld, namely by associating the indicated directory with the indicated keyword for purposes of the :dir argument. By the ``indicated directory'' we mean, in the case that the pathname is a relative pathname, the directory relative to the current connected book directory; see cbd. See delete-include-book-dir for how to undo this effect.

A keyword that is already associated with a directory string by an existing invocation of add-include-book-dir cannot be associated with a different directory string. If that is your intention, first apply delete-include-book-dir to that keyword; see delete-include-book-dir. If however the new directory string is identical with the old, then the call of add-include-book-dir will be redundant (see redundant-events).

The keyword :system can never be redefined. It will always point to the absolute pathname of the system books directory, which by default is immediately under the directory where the ACL2 executable was originally built (see include-book, in particular the discussion there of ``books directory'').

This macro generates (in essence) a call (table acl2-defaults-table :include-book-dir-alist ...) and hence is local to any books and encapsulate events in which it occurs. See acl2-defaults-table. Even if you invoke add-include-book-dir before certifying a book, so that this event is among the book's portcullis commands rather than in the book itself, nevertheless that add-include-book-dir event will not be visible after the book is included. (Note: The above behavior is generally preserved in raw-mode (see set-raw-mode),though by means other than a table.)