fix CMake build errors for certain SDL2 installations

CMakeLists.txt

@@ -157,8 +157,14 @@ if (ENABLE_SDL2)
         target_include_directories(SDL2 INTERFACE "${SDL2_INCLUDE_DIR}")
     else()
         find_package(SDL2 REQUIRED)
-        include_directories(${SDL2_INCLUDE_DIRS})
 
+        # Some installations don't set SDL2_LIBRARIES
+        if("${SDL2_LIBRARIES}" STREQUAL "")
+            message(WARNING "SDL2_LIBRARIES wasn't set, manually setting to SDL2::SDL2")
+            set(SDL2_LIBRARIES "SDL2::SDL2")
+        endif()
+
+        include_directories(${SDL2_INCLUDE_DIRS})
         add_library(SDL2 INTERFACE)
         target_link_libraries(SDL2 INTERFACE "${SDL2_LIBRARIES}")
     endif()