CMake: Support SDL audio back-end

Rename UNIX_SOUND_SRC_SDL to UNIX_SOUND_SDL_SRC to match the
signature of other variables (ending with _HDR or _SRC).

build/cmake/lib/core/CMakeLists.txt

@@ -15,6 +15,9 @@ if(WIN32)
     wx_append_sources(CORE_SRC BASE_AND_GUI_WIN32)
 elseif(UNIX)
     wx_append_sources(CORE_SRC UNIX)
+    if(wxUSE_LIBSDL)
+        wx_append_sources(CORE_SRC UNIX_SOUND_SDL)
+    endif()
 endif()
 
 if(WXMSW)
@@ -84,5 +87,14 @@ if(WXGTK AND wxUSE_PRIVATE_FONTS)
         ${FONTCONFIG_LIBRARIES}
     )
 endif()
+if(UNIX AND wxUSE_LIBSDL)
+    if(SDL2_FOUND)
+        wx_lib_include_directories(core PUBLIC ${SDL2_INCLUDE_DIR})
+        wx_lib_link_libraries(core PUBLIC ${SDL2_LIBRARY})
+    elseif(SDL_FOUND)
+        wx_lib_include_directories(core PUBLIC ${SDL_INCLUDE_DIR})
+        wx_lib_link_libraries(core PUBLIC ${SDL_LIBRARY})
+    endif()
+endif()
 
 wx_finalize_lib(core)