Workaround to bypass library's BuildConfig.DEBUG (always true, always release build type) via custom variable

app_build.gradle

// Application
apply plugin: 'android'

repositories {
    mavenCentral()
}

android {
    compileSdkVersion rootProject.ext.compileSdkVersion
    buildToolsVersion rootProject.ext.buildToolsVersion

    compileOptions {
        sourceCompatibility JavaVersion.VERSION_1_7
        targetCompatibility JavaVersion.VERSION_1_7
    }

    defaultConfig {
        minSdkVersion rootProject.ext.minSdkVersion
        targetSdkVersion rootProject.ext.targetSdkVersion
        versionCode 1
        versionName "1.0 alpha"

        rootProject.ext.variantRelease = false //default  we are in debug mode, will be overriden on task execution
    }

    buildTypes {
        release {
            runProguard false
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.txt'
        }
        debug {
            versionNameSuffix ' dev'
        }
    }

}

dependencies {
    // ui
    compile 'com.android.support:support-v4:+'
    // local jars
    compile fileTree(dir: 'libs', include: ['*.jar'])
}

// Trigger build type (as soon as possible) and make some action via corresponding tasks

project.afterEvaluate {
    tasks.all { task ->
        if (task.name =~ /check.*Manifest/) {
            if (task.name =~ /[dD]ebug/) {
                task.dependsOn(onDebugTypeTriggered)
            } else {
                task.dependsOn(onReleaseTypeTriggered)
            }
        }
    }
}

task onDebugTypeTriggered << {
    rootProject.ext.variantRelease = false
}

task onReleaseTypeTriggered << {
    rootProject.ext.variantRelease = true
}

lib_build.gradle

// Library
apply plugin: 'android-library'

repositories {
    mavenCentral()
}

android {
    compileSdkVersion rootProject.ext.compileSdkVersion
    buildToolsVersion rootProject.ext.buildToolsVersion

    compileOptions {
        sourceCompatibility JavaVersion.VERSION_1_7
        targetCompatibility JavaVersion.VERSION_1_7
    }

    defaultConfig {
        minSdkVersion rootProject.ext.minSdkVersion
        targetSdkVersion rootProject.ext.targetSdkVersion
    }

}

// Trigger BuildConfig creation

project.afterEvaluate {
    tasks.all { task ->
        if (task.name =~ /generate.*BuildConfig/) {
            task.dependsOn(propagateBuildConfig)
        }
    }
}

task propagateBuildConfig << {
    project.android.buildTypes.all { type ->
        type.buildConfigField "boolean", "RELEASE", isVariantRelease().toString()
    }
}

def isVariantRelease() {
    return rootProject.ext.variantRelease
}

This works only if you are satisfied doing one build type at a time /w clean. As soon as you try to compile 2 or more builds / flavors at once Gradle will cache the first "release" compilation of the subproject and use it for every subsequent build type. So if you compile debug and release at once using (gradlew clean assemble), then the debug version of the project will contain the release version of the library. This happens because when you compile the debug and release builds of the main project, the build type of the subproject remains the same (namely "release"), and Gradle feels it doesn't need to recompile.