Try to unify cmake file for different versions of SDL2.

CMakeLists.txt

@@ -77,10 +77,15 @@ ENDIF()
 OPTION(MAIKO_DISPLAY_SDL "Use SDL for display." ON)
 IF(MAIKO_DISPLAY_SDL)
   FIND_PACKAGE(SDL2 REQUIRED)
+  if(TARGET SDL2::SDL2)
+    set(SDLNAME SDL2::SDL2)
+  elseif(target SDL)
+    set(SDLNAME SDL2)
+  endif()
   SET(MAIKO_DISPLAY_SDL_DEFINITIONS
     "-DSDL"
   )
-  SET(MAIKO_DISPLAY_SDL_LIBRARIES SDL2::SDL2)
+  SET(MAIKO_DISPLAY_SDL_LIBRARIES ${SDLNAME})
   SET(MAIKO_DISPLAY_SDL_SRCS
     src/sdl.c
   )
@@ -424,7 +429,7 @@ IF(MAIKO_DISPLAY_SDL)
   # Tell it that the SDL launcher is available.
   TARGET_COMPILE_DEFINITIONS(lde PUBLIC ${MAIKO_DISPLAY_SDL_DEFINITIONS})
   # This is needed so that it can call XOpenDisplay.
-  TARGET_LINK_LIBRARIES(lde SDL2::SDL2)
+  TARGET_LINK_LIBRARIES(lde ${SDLNAME})
 ENDIF()
 
 ADD_EXECUTABLE(ldeether src/ldeether.c src/dlpi.c)