Add a vendored copy of SDL

vendored/sdl/cmake/test/jni/Android.mk

@@ -0,0 +1,11 @@
+LOCAL_PATH := $(call my-dir)
+
+include $(CLEAR_VARS)
+
+LOCAL_MODULE := main_gui_androidmk
+LOCAL_SRC_FILES := ../main_gui.c
+LOCAL_SHARED_LIBRARIES += SDL2
+include $(BUILD_SHARED_LIBRARY)
+
+$(call import-module,SDL2main)
+$(call import-module,SDL2)