To build Orion Viewer you will need:
    android-sdk-r23+
    gradle 2.1+

    for building native libs you also need
        android-ndk-r9d+
        git 1.9+
        linux make util for building mupdf 3rd party libs (cygwin or mingw for windows system)

Thirdpardy build scripted in 'thirdparty_build.gradle' file (details in 'orion-viewer/jni/README').
It should be executed one time to build native libs.

Main project build logic scripted in build.gradle.

Specify path to android-sdk in 'local.properties' (use 'local.properties.sample' as example).