Revert "Bug 573595 - AutoBuildJob following a search and replace might
run as system job"

This caused lot of JDT core test fails, see bug 574098

This reverts commit 4bca749b780c2a9221c9614490e636fe5d2de99b.

Change-Id: Iae221bc4dd5d40143ac5cc6c3add70021264152e
Reviewed-on: https://git.eclipse.org/r/c/platform/eclipse.platform.resources/+/181688
Tested-by: Platform Bot <platform-bot@eclipse.org>
Reviewed-by: Andrey Loskutov <loskutov@gmx.de>
diff --git a/bundles/org.eclipse.core.resources/src/org/eclipse/core/internal/events/AutoBuildJob.java b/bundles/org.eclipse.core.resources/src/org/eclipse/core/internal/events/AutoBuildJob.java
index a3be0ad..04177f2 100644
--- a/bundles/org.eclipse.core.resources/src/org/eclipse/core/internal/events/AutoBuildJob.java
+++ b/bundles/org.eclipse.core.resources/src/org/eclipse/core/internal/events/AutoBuildJob.java
@@ -98,9 +98,12 @@
 				wakeUp(delay);
 				break;
 			case NONE :
-				if (isAutoBuilding) {
-					schedule(delay);
+				try {
+					setSystem(!isAutoBuilding);
+				} catch (IllegalStateException e) {
+					//ignore - the job has been scheduled since we last checked its state
 				}
+				schedule(delay);
 				break;
 		}
 	}
@@ -220,7 +223,7 @@
 		boolean wasAutoBuilding = isAutoBuilding;
 		isAutoBuilding = preferences.getBoolean(ResourcesPlugin.PREF_AUTO_BUILDING);
 		//force a build if autobuild has been turned on
-		if (!wasAutoBuilding && isAutoBuilding) {
+		if (!forceBuild && !wasAutoBuilding && isAutoBuilding) {
 			forceBuild = true;
 			build(false);
 		}