Add SDL_MIXER feature flag for conditional audio mixing support

Introduce ENABLE_SDL_MIXER feature flag to conditionally compile
SDL2_mixer-dependent audio code. This allows SDL2 graphics support
without the problematic SDL2_mixer library in emscripten builds.

Changes:
1. Makefile:
   - Add ENABLE_SDL_MIXER feature flag (default: 1)
   - Update GNU make version requirement: 3.81+ for $(info) function
   - Disable SDL_MIXER for emscripten builds (override = 0)
   - Conditionally link SDL2_mixer only when ENABLE_SDL_MIXER=1
   - Set ENABLE_SDL_MIXER=0 when pkg-config check fails

2. src/syscall_sdl.c:
   - Wrap #include <SDL_mixer.h> with #if RV32_HAS(SDL_MIXER)
   - Wrap Mix_* variable declarations (mid, sfx_chunk)
   - Wrap Mix_* function calls in init_audio() and shutdown_audio()
   - Wrap audio handler functions (sfx_handler, music_handler,
     stop_music, set_music_volume, play_sfx, play_music)
   - Wrap pthread_create calls for audio threads
   - Wrap syscall handlers (PLAY_MUSIC, PLAY_SFX, SET_MUSIC_VOLUME,
     STOP_MUSIC)

Strategy: Set ENABLE_SDL_MIXER=0 in Makefile before mk/wasm.mk is
included, so the RV32_FEATURE_SDL_MIXER macro is defined as 0 for
emscripten builds. This makes syscall_sdl.c skip all Mix_* code at
compile time, avoiding linkage errors.

Root cause: emscripten-ports/SDL2_mixer (archived Jan 2024) has
unfixable compilation warnings in music_modplug.c. The port system
enforces -sSTRICT -Werror which cannot be overridden.

Result: Games (Doom/Quake) work without audio mixing. SDL2 graphics
support remains functional. Non-emscripten builds retain full audio.

Note: mk/wasm.mk is intentionally left unchanged. The SDL_MIXER
disablement happens in Makefile before mk/wasm.mk inclusion.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: jserv

Makefile

@@ -180,6 +180,7 @@ ENABLE_FULL4G ?= 0
 
 # Experimental SDL oriented system calls
 ENABLE_SDL ?= 1
+ENABLE_SDL_MIXER ?= 1
 ifneq ("$(CC_IS_EMCC)", "1") # note that emcc generates port SDL headers/library, so it does not requires system SDL headers/library
 ifeq ($(call has, SDL), 1)
 ifeq (, $(shell which sdl2-config))
@@ -189,18 +190,30 @@ endif
 ifeq (1, $(shell pkg-config --exists SDL2_mixer; echo $$?))
 $(warning No SDL2_mixer lib installed. Check SDL2_mixer installation in advance)
 override ENABLE_SDL := 0
+override ENABLE_SDL_MIXER := 0
 endif
 endif
+else
+# Disable SDL_MIXER for emscripten builds to avoid SDL2_mixer port compilation issues
+# The emscripten-ports/SDL2_mixer was archived in Jan 2024 with unfixable warnings
+override ENABLE_SDL_MIXER := 0
+endif
 $(call set-feature, SDL)
+$(call set-feature, SDL_MIXER)
 ifeq ($(call has, SDL), 1)
 OBJS_EXT += syscall_sdl.o
+ifneq ("$(CC_IS_EMCC)", "1")
 $(OUT)/syscall_sdl.o: CFLAGS += $(shell sdl2-config --cflags)
+endif
 # 4 GiB of memory is required to run video games.
 ENABLE_FULL4G := 1
+ifneq ("$(CC_IS_EMCC)", "1")
 LDFLAGS += $(shell sdl2-config --libs) -pthread
+ifeq ($(call has, SDL_MIXER), 1)
 LDFLAGS += $(shell pkg-config --libs SDL2_mixer)
 endif
 endif
+endif
 
 # If SYSTEM is enabled and ELF_LOADER is not, then skip FULL4G bacause guestOS
 # has dedicated memory mapping range.

src/syscall_sdl.c

@@ -15,7 +15,9 @@
 #include <unistd.h>
 
 #include <SDL.h>
+#if RV32_HAS(SDL_MIXER)
 #include <SDL_mixer.h>
+#endif
 
 #include "riscv.h"
 #include "riscv_private.h"
@@ -94,11 +96,13 @@ typedef struct sound {
 /* SDL-mixer-related and music-related variables */
 static pthread_t music_thread;
 static uint8_t *music_midi_data;
+#if RV32_HAS(SDL_MIXER)
 static Mix_Music *mid;
 
 /* SDL-mixer-related and sfx-related variables */
 static pthread_t sfx_thread;
 static Mix_Chunk *sfx_chunk;
+#endif
 static uint8_t *sfx_samples;
 static uint32_t nr_sfx_samples;
 static int chan;
@@ -718,6 +722,7 @@ uint8_t *mus2midi(uint8_t *data, int *length)
     return midi_data;
 }
 
+#if RV32_HAS(SDL_MIXER)
 static void *sfx_handler(void *arg)
 {
     sound_t *sfx = (sound_t *) arg;
@@ -836,8 +841,10 @@ static void play_sfx(riscv_t *rv)
         .size = sfx_data_size,
         .volume = volume,
     };
+#if RV32_HAS(SDL_MIXER)
     pthread_create(&sfx_thread, NULL, sfx_handler, &sfx);
     sfx_thread_init = true;
+#endif
     /* FIXME: In web browser runtime, web workers in thread pool do not reap
      * after sfx_handler return, thus we have to join them. sfx_handler does not
      * contain infinite loop,so do not worry to be stalled by it */
@@ -893,8 +900,10 @@ static void play_music(riscv_t *rv)
         .looping = looping,
         .volume = volume,
     };
+#if RV32_HAS(SDL_MIXER)
     pthread_create(&music_thread, NULL, music_handler, &music);
     music_thread_init = true;
+#endif
     /* FIXME: In web browser runtime, web workers in thread pool do not reap
      * after music_handler return, thus we have to join them. music_handler does
      * not contain infinite loop,so do not worry to be stalled by it */
@@ -916,6 +925,7 @@ static void set_music_volume(riscv_t *rv)
     /* multiplied by 8 because volume's max is 15 */
     Mix_VolumeMusic(volume * 8);
 }
+#endif /* RV32_HAS(SDL_MIXER) */
 
 static void init_audio(void)
 {
@@ -933,6 +943,7 @@ static void init_audio(void)
         exit(EXIT_FAILURE);
     }
 
+#if RV32_HAS(SDL_MIXER)
     /* Initialize SDL2 Mixer */
     if (Mix_Init(MIX_INIT_MID) != MIX_INIT_MID) {
         rv_log_fatal("Mix_Init failed: %s", Mix_GetError());
@@ -943,6 +954,7 @@ static void init_audio(void)
         Mix_Quit();
         exit(EXIT_FAILURE);
     }
+#endif
     audio_init = true;
 }
 
@@ -956,6 +968,7 @@ static void shutdown_audio()
      * on a valid pthread_t identifier.
      */
 
+#if RV32_HAS(SDL_MIXER)
     if (music_thread_init) {
         stop_music();
         pthread_join(music_thread, NULL);
@@ -974,6 +987,7 @@ static void shutdown_audio()
 
     Mix_CloseAudio();
     Mix_Quit();
+#endif
 
     audio_init = sfx_thread_init = music_thread_init = false;
 }
@@ -1020,16 +1034,24 @@ void syscall_control_audio(riscv_t *rv)
 
     switch (request) {
     case PLAY_MUSIC:
+#if RV32_HAS(SDL_MIXER)
         play_music(rv);
+#endif
         break;
     case PLAY_SFX:
+#if RV32_HAS(SDL_MIXER)
         play_sfx(rv);
+#endif
         break;
     case SET_MUSIC_VOLUME:
+#if RV32_HAS(SDL_MIXER)
         set_music_volume(rv);
+#endif
         break;
     case STOP_MUSIC:
+#if RV32_HAS(SDL_MIXER)
         stop_music();
+#endif
         break;
     default:
         rv_log_error("Unknown sound control request: %d", request);