minor mac changes

[John Clements <clements@cs.rice.edu>]

1) For those building Mac binaries under OS 8&9, the location of the 
cw.sit file has changed.  It is now located in plt/src/mac, and must 
be expanded in place.  The project files contained in the unstuffed 
directory may be used to build version 199 (& later).

2) work proceeds apace on the OS X version of MrEd.  However, the OS 
X version of MzScheme is now running. To compile it, check out the 
v200-tagged sources, and compile as you would on any other UNIX 
installation (that is, run ./configure in the src directory, then 
'make mz' and 'make mzinstall')

john clements