BuildConfig.DEBUG always false when building library projects with gradle

This is expected behavior for this.

Library projects only publish their release variants for consumption by other projects or modules.

We're working at fixing this but this is non trivial and requires a significant amount of work.

You can track the issue at https://code.google.com/p/android/issues/detail?id=52962

With Android Studio 1.1 and having also the gradle version at 1.1 it is possible:

Library

android {
    publishNonDefault true
}

App

dependencies {
    releaseCompile project(path: ':library', configuration: 'release')
    debugCompile project(path: ':library', configuration: 'debug')
}

Complete documentation can be found here http://tools.android.com/tech-docs/new-build-system/user-guide#TOC-Library-Publication

EDIT:

The issue has just been marked as fixed for the Android Studio Gradle Version 3.0. There you can just use implementation project(path: ':library') and it'll select the correct configuration automatically.