Extended SDL Sound functions

This implements the extended GLK sound functions:

glk_schannel_create_ext()
glk_schannel_play_multi()
glk_schannel_set_volume_ext()
glk_schannel_pause()
glk_schannel_unpause()

Sound and volume notifications are still not working properly.
See bug report #204 and pull request #337.

These changes only touch the SDL sound backend.

I've made a test program here:
 https://github.com/angstsmurf/soundtest/releases

garglk/Jamfile

@@ -20,6 +20,8 @@ if $(OS) = MINGW
 
 if $(USESDL) = yes
 {
+    SubDirCcFlags
+        -DGARGLK_USESDL ;
     SubDirHdrs $(TOP) support sdl ;
     SubDirHdrs $(TOP) support sdl_sound ;
 }

garglk/cggestal.c

@@ -110,7 +110,7 @@ glui32 glk_gestalt_ext(glui32 id, glui32 val, glui32 *arr,
             return gli_conf_sound;
 
         case gestalt_Sound2:
-            return FALSE;
+            return gli_conf_sound;
 
         case gestalt_Unicode:
             return TRUE;

garglk/garglk.h

@@ -30,6 +30,10 @@
 
 #include <stddef.h>
 
+#ifdef GARGLK_USESDL
+#include <SDL_timer.h>
+#endif
+
 #include "gi_dispa.h"
 
 /* First, we define our own TRUE and FALSE and NULL, because ANSI
@@ -565,6 +569,18 @@ struct glk_schannel_struct
     int notify;
     int buffered;
 
+#ifdef GARGLK_USESDL
+    int paused;
+
+    /* for volume fades */
+    int volume_notify;
+    int volume_timeout;
+    int target_volume;
+    double float_volume;
+    double volume_delta;
+    SDL_TimerID timer;
+#endif
+
     gidispatch_rock_t disprock;
     channel_t *chain_next, *chain_prev;
 };