Allow custom compileSdk, buildToolsVersion, and targetSdkVersion

Author: Stephen Potter

android/build.gradle

@@ -9,13 +9,17 @@ buildscript {
 
 apply plugin: 'com.android.library'
 
+def DEFAULT_COMPILE_SDK_VERSION = 25
+def DEFAULT_BUILD_TOOLS_VERSION = "25.0.2"
+def DEFAULT_TARGET_SDK_VERSION = 25
+
 android {
-    compileSdkVersion 25
-    buildToolsVersion "25.0.2"
+    compileSdkVersion project.hasProperty('compileSdkVersion') ? project.compileSdkVersion : DEFAULT_COMPILE_SDK_VERSION
+    buildToolsVersion project.hasProperty('buildToolsVersion') ? project.buildToolsVersion : DEFAULT_BUILD_TOOLS_VERSION
 
     defaultConfig {
         minSdkVersion 16
-        targetSdkVersion 25
+        targetSdkVersion project.hasProperty('targetSdkVersion') ? project.targetSdkVersion : DEFAULT_TARGET_SDK_VERSION
         versionCode 1
         versionName "1.0"
         ndk {