[APU] Add ALSA apu on Linux

src/xenia/app/premake5.lua

@@ -101,6 +101,7 @@ project("xenia-app")
 
   filter("platforms:Linux")
     links({
+      "xenia-apu-alsa",
       "X11",
       "xcb",
       "X11-xcb",

src/xenia/app/xenia_main.cc

@@ -36,6 +36,9 @@
 
 // Available audio systems:
 #include "xenia/apu/nop/nop_audio_system.h"
+#if XE_PLATFORM_LINUX
+#include "xenia/apu/alsa/alsa_audio_system.h"
+#endif  // XE_PLATFORM_LINUX
 #if !XE_PLATFORM_ANDROID
 #include "xenia/apu/sdl/sdl_audio_system.h"
 #endif  // !XE_PLATFORM_ANDROID
@@ -60,11 +63,23 @@
 #include "xenia/hid/xinput/xinput_hid.h"
 #endif  // XE_PLATFORM_WIN32
 
-DEFINE_string(apu, "any", "Audio system. Use: [any, nop, sdl, xaudio2]", "APU");
-DEFINE_string(gpu, "any", "Graphics system. Use: [any, d3d12, vulkan, null]",
-              "GPU");
-DEFINE_string(hid, "any", "Input system. Use: [any, nop, sdl, winkey, xinput]",
-              "HID");
+#if XE_PLATFORM_WIN32
+#define APU_OPTIONS "[any, nop, sdl, xaudio2]"
+#define GPU_OPTIONS "[any, d3d12, vulkan, null]"
+#define HID_OPTIONS "[any, nop, sdl, winkey, xinput]"
+#elif XE_PLATFORM_LINUX
+#define APU_OPTIONS "[any, alsa, nop, sdl]"
+#define GPU_OPTIONS "[any, vulkan, null]"
+#define HID_OPTIONS "[any, nop, sdl]"
+#else
+#define APU_OPTIONS "[any, nop, sdl]"
+#define GPU_OPTIONS "[any, vulkan, null]"
+#define HID_OPTIONS "[any, nop, sdl]"
+#endif
+
+DEFINE_string(apu, "any", "Audio system. Use: " APU_OPTIONS, "APU");
+DEFINE_string(gpu, "any", "Graphics system. Use: " GPU_OPTIONS, "GPU");
+DEFINE_string(hid, "any", "Input system. Use: " HID_OPTIONS, "HID");
 
 DEFINE_path(
     storage_root, "",
@@ -296,6 +311,9 @@ std::unique_ptr<apu::AudioSystem> EmulatorApp::CreateAudioSystem(
 #if XE_PLATFORM_WIN32
   factory.Add<apu::xaudio2::XAudio2AudioSystem>("xaudio2");
 #endif  // XE_PLATFORM_WIN32
+#if XE_PLATFORM_LINUX
+  factory.Add<apu::alsa::ALSAAudioSystem>("alsa");
+#endif  // XE_PLATFORM_LINUX
 #if !XE_PLATFORM_ANDROID
   factory.Add<apu::sdl::SDLAudioSystem>("sdl");
 #endif  // !XE_PLATFORM_ANDROID