Use SDL_TimerID properly.

This involves a GARGLK_USESDL macro to conditionally include
SDL_timer.h and the SDL-specific members of glk_schannel_struct
only when compiling with SDL sound support.

Multi-line description of commit,
feel free to be detailed.

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/garglk.h

@@ -32,6 +32,10 @@
 
 #include "gi_dispa.h"
 
+#ifdef GARGLK_USESDL
+#include "SDL/SDL_timer.h"
+#endif
+
 /* First, we define our own TRUE and FALSE and NULL, because ANSI
  * is a strange world.
  */
@@ -564,6 +568,8 @@ struct glk_schannel_struct
     glui32 loop;
     int notify;
     int buffered;
+
+#ifdef GARGLK_USESDL
     int paused;
 
     /* for volume fades */
@@ -572,7 +578,8 @@ struct glk_schannel_struct
     int target_volume;
     double float_volume;
     double volume_delta;
-    int timer;
+    SDL_TimerID timer;
+#endif
 
     gidispatch_rock_t disprock;
     channel_t *chain_next, *chain_prev;

garglk/sndsdl.c

@@ -190,7 +190,7 @@ static void cleanup_channel(schanid_t chan)
     chan->music = 0;
 
     if (chan->timer)
-        SDL_RemoveTimer((SDL_TimerID)chan->timer);
+        SDL_RemoveTimer(chan->timer);
 
     chan->timer = 0;
 }
@@ -321,7 +321,7 @@ Uint32 volume_timer_callback(Uint32 interval, void *param)
             gli_strict_warning("volume_timer_callback: invalid timer.");
             return 0;
         }
-        SDL_RemoveTimer((SDL_TimerID)chan->timer);
+        SDL_RemoveTimer(chan->timer);
         chan->timer = 0;
 
         if (chan->volume != chan->target_volume)
@@ -358,9 +358,9 @@ void init_fade(schanid_t chan, int volume, int duration, int notify)
     chan->volume_timeout = FADE_GRANULARITY;
 
     if (chan->timer)
-        SDL_RemoveTimer((SDL_TimerID)chan->timer);
+        SDL_RemoveTimer(chan->timer);
 
-    chan->timer = (int)SDL_AddTimer((Uint32)(duration / FADE_GRANULARITY), volume_timer_callback, (void *)chan);
+    chan->timer = SDL_AddTimer((Uint32)(duration / FADE_GRANULARITY), volume_timer_callback, (void *)chan);
 
     if (!chan->timer)
     {