cmake: wrap SDL_STATIC_PIC in double quotes (aka never trust user input)

Suggested by @Wohlstand

CMakeLists.txt

@@ -3001,7 +3001,7 @@ if(SDL_STATIC)
   else()
     set_target_properties(SDL2-static PROPERTIES OUTPUT_NAME "SDL2")
   endif()
-  set_target_properties(SDL2-static PROPERTIES POSITION_INDEPENDENT_CODE ${SDL_STATIC_PIC})
+  set_target_properties(SDL2-static PROPERTIES POSITION_INDEPENDENT_CODE "${SDL_STATIC_PIC}")
   # Note: The clang toolset for Visual Studio does not support /NODEFAULTLIB.
   if(MSVC AND NOT SDL_LIBC AND NOT MSVC_CLANG AND NOT CMAKE_GENERATOR_PLATFORM STREQUAL "ARM")
     set_target_properties(SDL2-static PROPERTIES LINK_FLAGS_RELEASE "/NODEFAULTLIB")