Add initial libretro implementation

src/duckstation-libretro/CMakeLists.txt

@@ -0,0 +1,23 @@
+add_library(duckstation-libretro SHARED
+  libretro_audio_stream.cpp
+  libretro_audio_stream.h
+  libretro_host_display.cpp
+  libretro_host_display.h
+  libretro_host_interface.cpp
+  libretro_host_interface.h
+  libretro_settings_interface.cpp
+  libretro_settings_interface.h
+  main.cpp
+  opengl_host_display.cpp
+  opengl_host_display.h
+)
+
+if(WIN32)
+  target_sources(duckstation-libretro PRIVATE
+    d3d11_host_display.cpp
+    d3d11_host_display.h
+  )
+endif()
+
+target_link_libraries(duckstation-libretro PRIVATE core common imgui glad scmversion libretro-common)
+