fix: avoid SDL3 target conflict when a system SDL3 is installed (#22)

find_package(SDL3) was followed by a check of INTERFACE_INCLUDE_DIRECTORIES
on SDL3::SDL3 to decide whether the found package is usable. In the standard
SDL3 package layout, SDL3::SDL3 is an alias of SDL3::SDL3-shared, which gets
its include dirs transitively via SDL3::Headers instead of carrying them
itself. The check therefore came back empty on e.g. Homebrew installs, SDL3
was fetched from source anyway, and its add_library(SDL3::Headers ALIAS ...)
collided with the already-imported target:

  CMake Error: add_library cannot create ALIAS target "SDL3::Headers"
  because another target with the same name already exists.

Trust the find_package result instead: fetch only when no SDL3::SDL3 target
exists.

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

Author: MaxHeimbrock

cmake/ExampleDeps.cmake

@@ -10,21 +10,10 @@ if(NOT TARGET nlohmann_json::nlohmann_json)
 endif()
 
 find_package(SDL3 CONFIG QUIET)
-set(_need_sdl3_fetch TRUE)
-if(TARGET SDL3::SDL3)
-  get_target_property(_sdl3_include_dirs SDL3::SDL3 INTERFACE_INCLUDE_DIRECTORIES)
-  if(_sdl3_include_dirs)
-    set(_need_sdl3_fetch FALSE)
-  endif()
-endif()
-
-if(_need_sdl3_fetch)
+if(NOT TARGET SDL3::SDL3)
   FetchContent_Declare(
     SDL3
     URL https://github.com/libsdl-org/SDL/releases/download/release-3.2.26/SDL3-3.2.26.tar.gz
   )
   FetchContent_MakeAvailable(SDL3)
 endif()
-
-unset(_need_sdl3_fetch)
-unset(_sdl3_include_dirs)