Consistently use /Z7 for archive targets to get debug symbols into
consumers.  On Mac, bring the libz symbols file that was started
in freetype into this repo where it belongs.  These are two
files, manually maintained, that list the global symbols defined
in the libz.a archives (debug and release).  These symbol lists
can then be used in an --unexported-symbols option to prevent
re-export in consumer shared libraries.