update JNI build instructions

jni/BUILD_JNI.txt

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+#!/bin/bash
+
+# Build native libraries:
+# - modify jni/builder/JniBuilder.java for your platform.
+# - get gdx-jnigen, then run:
+
+javac -cp ../vtm-ext-libs/gdx/gdx-jnigen.jar jni/builder/JniBuilder.java
+java -cp ../vtm-ext-libs/gdx/gdx-jnigen.jar:../vtm-ext-libs/gdx/gdx-jnigen-sources.jar:jni/builder JniBuilder
+
+## to build android:
+export ANDROID_HOME=/home/jeff/android-sdk
+export NDK_HOME=/home/jeff/android-ndk-r8d
+
+## compile
+cd jni
+ant -v
+
+## copy vtm/libs/armeabi* to vtm-android/libs
+rm -rf ../../vtm-ext-libs/vtm-android/libs/*
+cp -r ../libs/armeabi* ../../vtm-ext-libs/vtm-android/libs/
+
+

jni/Makefile

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+all:
+