Build native code for multiple architectures.

* Include native libs without requiring intermediate .jar.
 * Fix build warnings; use latest build tools.

BUILDING.md

@@ -6,3 +6,17 @@ Building TextSecure
 Execute Gradle:
 
     ./gradlew build
+
+Re-building native components
+-----------------------------
+
+Note: This step is optional; native components are contained as binaries (see [library/libs](library/libs)).
+
+1. Ensure that the Android NDK is installed.
+
+Execute ndk-build:
+
+    cd library
+    ndk-build
+
+Afterwards, execute Gradle as above to re-create the APK.