[plt-scheme] Mz/Mr build changes in CVS, both exp and v299

Some changes in CVS:

 - The Boehm conservative collector is now version 6.3 (instead of
   6.1alpha1) for both the exp-tagged and v299-tagged sources

 - For v299 under Unix, the default configure mode is to enable shared
   libraries. Thus the binaries are "mzscheme" and "mred" plus the
   libraries "libmzscheme-<vers>.so", "libmzgc-<vers>.so", and
   "libmred-<vers>.so". The libraries are installed into plt/lib.

   I'm not sure that this is a good choice. For example, it makes
   moving the "plt" directory more difficult. But it's more consistent
   with the Windows and Mac builds (which always use DLLs and shared
   libraries), and I know that many people configure with
   --enable-shared anyway. Let me know if you have an opinion on this.

   MzScheme3m and MrEd3m v299 now build in --enable-shared mode, too.
   The shared libraries are "libmzscheme3m-<vers>.so" and
   "lib-mred3m-<vers>.so".

Matthew