Allow the override of compileSdkVersion, buildToolsVersion, minSdkVersion and targetSdkVersion

Author: James Pearson

android/build.gradle

@@ -1,12 +1,19 @@
 apply plugin: 'com.android.library'
 
+
+def safeExtGet(prop, fallback) {
+    rootProject.ext.has(prop) ? rootProject.ext.get(prop) : fallback
+}
+
+
 android {
-  compileSdkVersion 23
-  buildToolsVersion "23.0.1"
+  compileSdkVersion safeExtGet("compileSdkVersion", 23)
+  buildToolsVersion safeExtGet("buildToolsVersion", "23.0.1")
+
 
   defaultConfig {
-    minSdkVersion 16
-    targetSdkVersion 23
+    minSdkVersion safeExtGet("minSdkVersion", 16)
+    targetSdkVersion safeExtGet("targetSdkVersion", 23)
     versionCode 1
     versionName "1.0"
     ndk {