take initial stab at macsound interface

Use macOS AppKit framework routines for a first cut at a
macsound soundlib interface.

Requires WANT_MACSOUND=1 on build.

Nothing has been done to move the stock sounds into the resources
of a bundle, so after building, if you want to try the stock sounds
out:
	cp sound/wav/*.wav ~/Library/Sounds

Because the NSSound macOS routines always do the search, supposedly
the following locations are searched in this order:
   1. the application’s main bundle
   2. ~/Library/Sounds
   3. /Library/Sounds
   4. /Network/Library/Sounds
   5. /System/Library/Sounds

Although not specifically implemented as of yet, it may be pretty
close to being able to put soundeffects wav files (by se_ name)
into ~/Library/Sounds working for the SND_SOUNDEFFECTS_AUTOMAP feature.

Feedback is welcome. Contributions for improving it are even more
welcome.

The new soundlib supporting file is named
sound/macsound/macsound.m since it's got objective C in it.

Known bugs and glitches:

The Hero_playnotes on a set of 5 notes goes too fast, so there
needs to be a slight delay added between the note of a multi-note
play.

src/decl.c

@@ -207,7 +207,7 @@ const struct instance_globals_a g_init_a = {
     /* shk.c */
     FALSE, /* auto_credit */
     /* sounds.c */
-    soundlib_unassigned, /* enum soundlib_ids active_soundlib */
+    soundlib_nosound, /* enum soundlib_ids active_soundlib */
 
     /* trap.c */
     { 0, 0, FALSE }, /* acid_ctx */