Android: speed up rebuild, and prevent race condition fail

Speeding up rebuild:

Before, we've written the android_version.h file at every rebuild.
As it thought the file's content has changed, make has rebuilt files
in the source tree that depended on that header file during rebuild,
causing lots of files being rebuilt without there being a reason.

The reasoning of make can be observed by passing -d and V=1 params
to the ndk-build command. You then got logging entries like:

Prerequisite `jni/src/android_version.h' is newer than target `obj/local/armeabi-v7a-hard/objs-debug/minetest/jni/src/areastore.o'.

Preventing race condition build fail:

Before, there was a race condition, where, if the prep_srcdir target
was executed in parallel with the $(ROOT)/jni/src/android_version.h
one, it could happen that the jni/src directory was nonexistent, and
we were trying to write into a file inside. This resulted in a build
failue:

/bin/sh: 1: cannot create [...]/jni/src/android_version.h: Directory nonexistent

Additionally, we now don't remove the link to src/ needlessly anymore.
Keeping this wouldn't have affected the rebuild, but this way its
more proper.