Fix SDL autodetection

Moving to the official SDL2 config resulted in a few variables now following
the standard CMake naming conventions. Use the new names when checking whether
SDL is present.

cmake/FreecivDependencies.cmake

@@ -65,17 +65,16 @@ if(FREECIV_ENABLE_NLS)
 endif()
 
 # SDL2 for audio
-find_package(SDL2 QUIET)
-find_package(SDL2_mixer QUIET)
-if (SDL2_MIXER_LIBRARIES AND SDL2_LIBRARY)
+find_package(SDL2)
+find_package(SDL2_mixer)
+if (SDL2_MIXER_LIBRARIES AND SDL2_FOUND)
   set(AUDIO_SDL TRUE)
 endif()
-if (NOT SDL2_LIBRARY)
-  message("SDL2 not found")
-  set(SDL2_INCLUDE_DIR "")
+if (NOT SDL2_FOUND)
+  message(STATUS "SDL2 not found - audio support disabled")
 endif()
 if (NOT SDL2_MIXER_LIBRARIES)
-  message("SDL2_mixer not found")
+  message(STATUS "SDL2_mixer not found - audio support disabled")
   set(SDL2_MIXER_LIBRARIES "")
   set(SDL2_MIXER_INCLUDE_DIR "")
 endif()