make sure there is a v7 directory in your sdk, I thought having the 'Android Support Library' (in Extras) was sufficient. Turns out I was missing the 'Local Maven repository for Support Libraries (extras;android;m2repository)' Studio found that actually and fixed  the gradle dependencies. using gradle to build then worked.
$ cat app/build.gradle
apply plugin: 'com.android.application'
android {
    compileSdkVersion 23
    buildToolsVersion "23.0.2"
    defaultConfig {
        applicationId "pat.example.com.gdbdemo"
        minSdkVersion 15
        targetSdkVersion 23
        versionCode 1
        versionName "1.0"
    }
    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
        }
    }
}
dependencies {
     compile fileTree(dir: 'libs', include: ['*.jar'])
    compile 'com.android.support:appcompat-v7:23.2.1'
}