Remove custom FindSDL2.cmake

SDL2 supports CMake natively and installs an SDL2Config.cmake file. Hence we do
not need a find module. The custom module was not handling linking to macOS
frameworks, which was hard coded into client CMakeLists.txt instead. Just
switch to the native module, which does all of this for us natively.

See #2075.

client/CMakeLists.txt

@@ -158,7 +158,6 @@ target_link_libraries(freeciv21-client PRIVATE tolua)
 
 target_link_libraries(freeciv21-client PRIVATE Qt5::Widgets)
 target_link_libraries(freeciv21-client PRIVATE Qt5::Svg)
-target_link_libraries(freeciv21-client PRIVATE ${SDL2_MIXER_LIBRARIES} ${SDL2_LIBRARY})
 
 if(NOT EMSCRIPTEN)
   target_sources(freeciv21-client PRIVATE servers.cpp)
@@ -168,13 +167,15 @@ if (AUDIO_SDL)
   target_sources(freeciv21-client PRIVATE audio/audio_sdl.cpp)
   if (NOT EMSCRIPTEN)
     # SDL comes with the -s option on emscripten
-    target_include_directories(freeciv21-client PRIVATE ${SDL2_INCLUDE_DIR})
+    # See https://github.com/microsoft/vcpkg/blob/2023.12.12/ports/sdl2/usage
+    target_link_libraries(freeciv21-client
+      PRIVATE $<IF:$<TARGET_EXISTS:SDL2::SDL2>,SDL2::SDL2,SDL2::SDL2-static>)
+    target_link_libraries(freeciv21-client PRIVATE ${SDL2_MIXER_LIBRARIES})
     target_include_directories(freeciv21-client PRIVATE ${SDL2_MIXER_INCLUDE_DIR})
   endif()
 endif()
 
-if(APPLE)
-  target_link_libraries(freeciv21-client PRIVATE "-framework AudioUnit" "-framework CoreAudio" "-framework ForceFeedback" "-framework AudioToolbox")
+if (APPLE)
   qt5_import_plugins(freeciv21-client INCLUDE Qt5::QCocoaIntegrationPlugin Qt5::QSvgPlugin)
 endif()