build: only update version file when it changed

Avoids recompiling jme3-core on every build

jme3-core/build.gradle

@@ -17,9 +17,9 @@ sourceSets {
     }
 }
 
-task updateVersionPropertiesFile << {
-    def verfile = file('src/main/resources/com/jme3/system/version.properties')
-    verfile.text = "# THIS IS AN AUTO-GENERATED FILE..\n" +
+task updateVersionPropertiesFile {
+    def versionFile = file('src/main/resources/com/jme3/system/version.properties')
+    def versionFileText = "# THIS IS AN AUTO-GENERATED FILE..\n" +
                           "# DO NOT MODIFY!\n" +
                           "build.date=${jmeBuildDate}\n" +
                           "git.revision=${jmeRevision}\n" +
@@ -31,9 +31,13 @@ task updateVersionPropertiesFile << {
                           "version.full=${jmeFullVersion}\n" +
                           "version.number=${jmeVersion}\n" +
                           "version.tag=${jmeVersionTag}"
-}
-
-compileJava.dependsOn(updateVersionPropertiesFile)
 
-dependencies {
+    outputs.upToDateWhen { 
+        versionFile.text == versionFileText
+    }
+    doLast {
+        versionFile.text = versionFileText
     }
+}
+
+processResources.dependsOn updateVersionPropertiesFile