Build: Use -release with java 9 instead of -source/-target

This uses the new -release flag for javac which does not require setting
the bootclasspath as -source/-target did.

build.gradle

@@ -144,6 +144,14 @@ subprojects {
       // see https://discuss.gradle.org/t/add-custom-javadoc-option-that-does-not-take-an-argument/5959
       javadoc.options.encoding='UTF8'
       javadoc.options.addStringOption('Xdoclint:all,-missing', '-quiet')
+      /*
+      TODO: building javadocs with java 9 b118 is currently broken with weird errors, so
+      for now this is commented out...try again with the next ea build...
+      javadoc.executable = new File(project.javaHome, 'bin/javadoc')
+      if (project.javaVersion == JavaVersion.VERSION_1_9) {
+        // TODO: remove this hack! gradle should be passing this...
+        javadoc.options.addStringOption('source', '8')
+      }*/
     }
   }
 

buildSrc/src/main/groovy/org/elasticsearch/gradle/BuildPlugin.groovy

@@ -143,7 +143,7 @@ class BuildPlugin implements Plugin<Project> {
             }
 
             project.rootProject.ext.javaHome = javaHome
-            project.rootProject.ext.javaVersion = javaVersion
+            project.rootProject.ext.javaVersion = javaVersionEnum
             project.rootProject.ext.buildChecksDone = true
         }
         project.targetCompatibility = minimumJava
@@ -387,10 +387,13 @@ class BuildPlugin implements Plugin<Project> {
                 options.encoding = 'UTF-8'
                 //options.incremental = true
 
-                // gradle ignores target/source compatibility when it is "unnecessary", but since to compile with
-                // java 9, gradle is running in java 8, it incorrectly thinks it is unnecessary
-                assert minimumJava == JavaVersion.VERSION_1_8
-                options.compilerArgs << '-target' << '1.8' << '-source' << '1.8'
+                if (project.javaVersion == JavaVersion.VERSION_1_9) {
+                    // hack until gradle supports java 9's new "-release" arg
+                    assert minimumJava == JavaVersion.VERSION_1_8
+                    options.compilerArgs << '-release' << '8'
+                    project.sourceCompatibility = null
+                    project.targetCompatibility = null
+                }
             }
         }
     }