Automatically updated share-subdirs-list



We currently have a function, share-subdir-list (in src/init-cl.lisp),
to compute the list of subdirectories of share, which eventually becomes
the list of share directories that maxima searches.

However, this only exists for a handful of Lisps like ccl (new), clisp,
cmucl, ecl, and sbcl.  For other lisps, we use a hand-edited list of
directories which has to be edited whenever a new share directory is
added.  This is error-prone.

One way to make this automatic is to have configure produce the list of
subdirectories and substitute that list into the list in
default-share-subdirs-list.  For this to work, we would need to move
init-cl.lisp to init-cl.lisp.in so that configure will process
init-cl.lisp.in to create init-cl.lisp, substituting in correct value of
into default-share-subdirs-list.  (Actually, we can create a new file,
share-subdirs.lisp.in, so we don't mess up init-cl.lisp.)

I've tested this and it produces the correct list.

In fact, with this change, we can get rid of all of the special versions
of share-subdir-list function, and just use default-share-subdirs-list. 
This is a win, I think.

Any objections?

Ray