To build Orion Viewer you will need:
android-sdk-r23+
gradle 1.12+
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).