Seems like you’re missing SDL_android_main.c from the compile? Located /jni/SDL/src/main/android. Referenced in /jni/src/Android.mk
Is there any reason why you’re not using ANT instead of Cmake?
Glad you solved your compile problem.
I use ANT along with all the Android glue supplied in the SDL2 source. Once I got my head around how to set it up, it works quite well.