cmake: only mark SDL_{SHARED,STATIC} as cache variables if they were already cache variables

CMakeLists.txt

@@ -390,8 +390,17 @@ set_option(SDL_CLANG_TIDY          "Run clang-tidy static analysis" OFF)
 
 set(SDL_VENDOR_INFO "" CACHE STRING "Vendor name and/or version to add to SDL_REVISION")
 
-cmake_dependent_option(SDL_SHARED "Build a shared version of the library" ${SDL_SHARED_DEFAULT} ${SDL_SHARED_AVAILABLE} OFF)
-cmake_dependent_option(SDL_STATIC "Build a static version of the library" ${SDL_STATIC_DEFAULT} ${SDL_STATIC_AVAILABLE} OFF)
+if(DEFINED CACHE{SDL_SHARED} OR DEFINED CACHE{SDL_STATIC})
+  cmake_dependent_option(SDL_SHARED "Build a shared version of the library" ${SDL_SHARED_DEFAULT} ${SDL_SHARED_AVAILABLE} OFF)
+  cmake_dependent_option(SDL_STATIC "Build a static version of the library" ${SDL_STATIC_DEFAULT} ${SDL_STATIC_AVAILABLE} OFF)
+else()
+  if(NOT DEFINED SDL_SHARED)
+    set(SDL_SHARED ${SDL_SHARED_DEFAULT})
+  endif()
+  if(NOT DEFINED SDL_STATIC)
+    set(SDL_STATIC ${SDL_STATIC_DEFAULT})
+  endif()
+endif()
 option(SDL_TEST_LIBRARY   "Build the SDL3_test library" ON)
 
 dep_option(SDL_TESTS           "Build the test directory" ${SDL3_MAINPROJECT} SDL_TEST_LIBRARY OFF)