buildscripts: define the Java compatibility levels in a single place

common.gradle

@@ -10,13 +10,8 @@ apply plugin: 'signing'
 group = 'org.jmonkeyengine'
 version = jmeFullVersion
 
-sourceCompatibility = '1.8'
-
-if(JavaVersion.current() >= JavaVersion.VERSION_1_9) {
-    compileJava {
-		options.release = 8
-    }
-}
+sourceCompatibility = JavaVersion.VERSION_1_8
+targetCompatibility = JavaVersion.VERSION_1_8
 
 tasks.withType(JavaCompile) { // compile-time options:
     //options.compilerArgs << '-Xlint:deprecation' // to show deprecation warnings

jme3-android-examples/build.gradle

@@ -25,11 +25,6 @@ android {
         }
     }
 
-    compileOptions {
-        sourceCompatibility JavaVersion.VERSION_1_8
-        targetCompatibility JavaVersion.VERSION_1_8
-    }
-
     sourceSets {
         main {
             java {

jme3-ios/build.gradle

@@ -1,6 +1,3 @@
-sourceCompatibility = JavaVersion.VERSION_1_8
-targetCompatibility = JavaVersion.VERSION_1_8
-
 dependencies {
     api project(':jme3-core')
     api project(':jme3-plugins')

jme3-lwjgl3/build.gradle

@@ -1,5 +1,3 @@
-sourceCompatibility = '1.8'
-
 dependencies {
     api project(':jme3-core')
     api project(':jme3-desktop')

jme3-vr/build.gradle

@@ -1,5 +1,3 @@
-sourceCompatibility = '1.8'
-
 dependencies {
     api project(':jme3-core')
     api project(':jme3-lwjgl3')