appcompat-v7:21.0.0′: No resource found that matches the given name: attr ‘android:actionModeShareDrawable’

This is likely because you haven’t set your compileSdkVersion to 21 in your build.gradle file. You also probably want to change your targetSdkVersion to 21.

android {
    //...
    compileSdkVersion 21

    defaultConfig {
        targetSdkVersion 21
    }
    //...
}

This requires you to have downloaded the latest SDK updates to begin with.

Once you’ve downloaded all the updates (don’t forget to also update the Android Support Library/Repository, too!) and updated your compileSdkVersion, re-sync your Gradle project.

Edit: For Eclipse or general IntelliJ users

See reVerse’s answer. He has a very thorough walk through!