Issue 4030 - Move JNI bindings into separate library project

libadblockplus-android/build.xml

 <?xml version="1.0" encoding="UTF-8"?>
 <project name="Adblock Plus Library" default="help">
 
     <!-- The local.properties file is created and updated by the 'android' tool.
          It contains the path to the SDK. It should *NOT* be checked into
          Version Control Systems. -->
     <property file="local.properties" />
 
     <!-- The ant.properties file can be created by you. It is only edited by the
          'android' tool to add properties to it.
@@ skipping 10 matching lines @@
 
          Properties related to the SDK location or the project target should
          be updated using the 'android' tool with the 'update' action.
 
          This file is an integral part of the build system for your
          application and should be checked into Version Control Systems.
 
          -->
     <property file="ant.properties" />
 
+    <fail
+        message="sdk.dir is missing. Make sure to generate specify it in local.properties or to inject it through an env var"
+        unless="sdk.dir"
+        />
+    <fail
+        message="ndk.dir is missing. Make sure to generate specify it in local.properties or to inject it through an env var"
+        unless="ndk.dir"
+        />
+
     <!-- The project.properties file is created and updated by the 'android'
          tool, as well as ADT.
 
          This contains project specific properties such as project target, and library
          dependencies. Lower level build properties are stored in ant.properties
          (or in .classpath for Eclipse projects).
 
          This file is an integral part of the build system for your
          application and should be checked into Version Control Systems. -->
     <loadproperties srcFile="project.properties" />
@@ skipping 27 matching lines @@
          ***********************
          ****** IMPORTANT ******
          ***********************
          In all cases you must update the value of version-tag below to read 'custom' instead of an integer,
          in order to avoid having your file be overridden by tools such as "android update project"
     -->
     <!-- version-tag: custom -->
     <import file="${sdk.dir}/tools/ant/build.xml" />
 
 </project>