cmake: do list(APPEND SDLMAIN_SOURCES)

CMakeLists.txt

@@ -1096,7 +1096,7 @@ if(ANDROID)
   endif()
 
   file(GLOB ANDROID_MAIN_SOURCES ${SDL2_SOURCE_DIR}/src/main/android/*.c)
-  set(SDLMAIN_SOURCES ${SDLMAIN_SOURCES} ${ANDROID_MAIN_SOURCES})
+  list(APPEND SDLMAIN_SOURCES ${ANDROID_MAIN_SOURCES})
 
   if(SDL_AUDIO)
     set(SDL_AUDIO_DRIVER_ANDROID 1)
@@ -2462,7 +2462,7 @@ elseif(VITA)
 
 elseif(PSP)
  file(GLOB PSP_MAIN_SOURCES ${SDL2_SOURCE_DIR}/src/main/psp/*.c)
- set(SDLMAIN_SOURCES ${SDLMAIN_SOURCES} ${PSP_MAIN_SOURCES})
+ list(APPEND SDLMAIN_SOURCES ${PSP_MAIN_SOURCES})
 
   if(SDL_AUDIO)
     set(SDL_AUDIO_DRIVER_PSP 1)