CMake: Fix X11 + SDL_mixer build

Stop using SDL2_FOUND to determine if SDL2 should be used or not,
because it may be defined as a side effect of
pkg_check_modules(SDL2_mixer).

CMakeLists.txt

@@ -71,10 +71,7 @@ else()
   optional_find_package(Freetype)
 
   include(FindPkgConfig)
-  if (NOT X11_FOUND)
-    optional_pkg_check_modules(SDL2 IMPORTED_TARGET sdl2)
-  endif()
-
+  optional_pkg_check_modules(SDL2 IMPORTED_TARGET sdl2)
   optional_pkg_check_modules(SDL2MIXER IMPORTED_TARGET SDL2_mixer)
   optional_pkg_check_modules(SDL2TTF IMPORTED_TARGET SDL2_ttf)
 
@@ -202,7 +199,7 @@ endif()
 
 if (EMSCRIPTEN OR ANDROID)
   set(SRC_JOYSTICK joystick_dmy.c)
-elseif (SDL2_FOUND)
+elseif (ENABLE_SDL)
   set(SRC_JOYSTICK joystick_sdl.c)
   set(SUMMARY_JOYSTICK "SDL")
   set(HAVE_SDLJOY 1)

src/CMakeLists.txt

@@ -93,7 +93,7 @@ else()  # non-emscripten, non-android
   if (FREETYPE_FOUND)
     target_link_libraries(xsystem35 PRIVATE Freetype::Freetype)
   endif()
-  if (SDL2_FOUND)
+  if (ENABLE_SDL)
     target_link_libraries(xsystem35 PRIVATE PkgConfig::SDL2)
   endif()
   if (SDL2MIXER_FOUND)