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.

sys/unix/unixmain.c

@@ -102,6 +102,18 @@ main(int argc, char *argv[])
 
     choose_windows(DEFAULT_WINDOW_SYS);
 
+#ifdef SND_LIB_INTEGRATED
+    /* One of the soundlib interfaces was integrated on build.
+     * We can leave a hint here for activate_chosen_soundlib later.
+     * assign_soundlib() just sets an indicator, it doesn't initialize
+     * any soundlib, and the indicator could be overturned before
+     * activate_chosen_soundlib() gets called.
+     */
+#if defined(SND_LIB_MACSOUND) && !defined(SND_LIB_QTSOUND)
+    assign_soundlib(soundlib_macsound);
+#endif
+#endif
+
 #ifdef CHDIR /* otherwise no chdir() */
     /*
      * See if we must change directory to the playground.