GRAILS-6484 - fix for class loading issue with ‘grails.doc.DocEngine’.

The script compiling class loader can't find ‘grails.doc.DocEngine’ for some unknown reason, even though it can find other classes from the same jar. To work around this we load the class using loadClass() on demand.

scripts/_GrailsDocs.groovy

@@ -271,7 +271,13 @@ def readDocProperties(DocPublisher publisher) {
 }
 
 def configureAliases() {
-    DocEngine.ALIAS.putAll(config.grails.doc.alias)
+    // See http://jira.codehaus.org/browse/GRAILS-6484 for why this is soft loaded
+    def docEngineClassName = "grails.doc.DocEngine"
+    def docEngineClass = classLoader.loadClass(docEngineClassName)
+    if (!docEngineClass) {
+        throw new IllegalStateException("Failed to load $docEngineClassName to configure documentation aliases")
+    }
+    docEngineClass.ALIAS.putAll(config.grails.doc.alias)
 }
 
 private readIfSet(DocPublisher publisher,String prop) {