Rebuild on any builder classpath change

Until now we only did a rebuild when the list of files on the builder
classpath changed. This ignored changes to the content of those files.
We now actively throw away the builder when the incremental inputs
indicate a change in the builder classpath.

Fixes #41

xtext-gradle-plugin/src/main/java/org/xtext/gradle/tasks/XtextGenerate.xtend

@@ -1,5 +1,6 @@
 package org.xtext.gradle.tasks;
 
+import com.google.common.base.Charsets
 import java.io.File
 import java.net.URLClassLoader
 import java.util.Collection
@@ -24,7 +25,6 @@ import org.xtext.gradle.protocol.GradleOutputConfig
 import org.xtext.gradle.protocol.IncrementalXtextBuilder
 import org.xtext.gradle.protocol.IncrementalXtextBuilderFactory
 import org.xtext.gradle.tasks.internal.FilteringClassLoader
-import com.google.common.base.Charsets
 
 class XtextGenerate extends DefaultTask {
 
@@ -73,12 +73,18 @@ class XtextGenerate extends DefaultTask {
 		val removedFiles = newLinkedHashSet
 		val outOfDateFiles = newLinkedHashSet
 		inputs.outOfDate[
-			if (getSources.contains(file) || getNullSafeClasspath.contains(file))
+			if (getSources.contains(file) || getNullSafeClasspath.contains(file)) {
 				outOfDateFiles += file
+			} else if (getXtextClasspath.contains(file)) {
+				closeBuilder
+			}
 		]
 		inputs.removed[
-			if (getSources.contains(file))
+			if (getSources.contains(file)) {
 				removedFiles += file
+			} else if (getXtextClasspath.contains(file)) {
+				closeBuilder
+			}
 		]
 
 		if (needsCleanBuild) {
@@ -172,10 +178,15 @@ class XtextGenerate extends DefaultTask {
 	}
 
 	private def initializeBuilder() {
+		closeBuilder
+		builder = new IncrementalXtextBuilderFactory().create(project.rootDir.path, languageSetups, nullSafeEncoding, builderClassLoader)
+	}
+
+	private def closeBuilder() {
 		if (builder !== null) {
 			(builder.class.classLoader as URLClassLoader).close
+			builder = null
 		}
-		builder = new IncrementalXtextBuilderFactory().create(project.rootDir.path, languageSetups, nullSafeEncoding, builderClassLoader)
 	}
 
 	private def isBuilderCompatible() {