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/init.cmake

@@ -268,4 +268,15 @@ if(wxUSE_GUI)
             wx_option_force_value(wxUSE_MEDIACTRL OFF)
         endif()
     endif()
+
+    if(UNIX AND wxUSE_LIBSDL)
+        find_package(SDL2)
+        if(NOT SDL2_FOUND)
+            find_package(SDL)
+        endif()
+        if(NOT SDL2_FOUND AND NOT SDL_FOUND)
+            message(WARNING "SDL not found, SDL Audio back-end won't be available")
+            wx_option_force_value(wxUSE_LIBSDL OFF)
+        endif()
+    endif()
 endif()