diff --git a/build/org.eclipse.cdt.managedbuilder.core.tests/tests/org/eclipse/cdt/managedbuilder/language/settings/providers/tests/GCCBuiltinSpecsDetectorTest.java b/build/org.eclipse.cdt.managedbuilder.core.tests/tests/org/eclipse/cdt/managedbuilder/language/settings/providers/tests/GCCBuiltinSpecsDetectorTest.java
index 4fc6d9b9946..976e00a9a2f 100644
--- a/build/org.eclipse.cdt.managedbuilder.core.tests/tests/org/eclipse/cdt/managedbuilder/language/settings/providers/tests/GCCBuiltinSpecsDetectorTest.java
+++ b/build/org.eclipse.cdt.managedbuilder.core.tests/tests/org/eclipse/cdt/managedbuilder/language/settings/providers/tests/GCCBuiltinSpecsDetectorTest.java
@@ -373,12 +373,16 @@ public class GCCBuiltinSpecsDetectorTest extends BaseTestCase {
 		detector.startupForLanguage(null);
 
 		detector.processLine(" "+loc+"/misplaced/include1");
+		detector.processLine("#undef BEFORE_INCLUDES");
 		detector.processLine("#include \"...\" search starts here:");
+		detector.processLine("#undef IN_THE_MIDDLE_OF_INCLUDES_0");
 		detector.processLine(" "+loc+"/local/include");
 		detector.processLine("#include <...> search starts here:");
 		detector.processLine(" "+loc+"/usr/include");
 		detector.processLine(" "+loc+"/usr/include/../include2");
+		detector.processLine("#undef IN_THE_MIDDLE_OF_INCLUDES_1");
 		detector.processLine(" "+loc+"/missing/folder");
+		detector.processLine("#undef IN_THE_MIDDLE_OF_INCLUDES_2");
 		detector.processLine(" "+loc+"/Library/Frameworks (framework directory)");
 		detector.processLine("End of search list.");
 		detector.processLine(" "+loc+"/misplaced/include2");
@@ -386,6 +390,7 @@ public class GCCBuiltinSpecsDetectorTest extends BaseTestCase {
 		detector.processLine(" "+loc+"/System/Library/Frameworks");
 		detector.processLine("End of framework search list.");
 		detector.processLine(" "+loc+"/misplaced/include3");
+		detector.processLine("#undef AFTER_INCLUDES");
 		detector.shutdownForLanguage();
 		detector.shutdown();
 
diff --git a/build/org.eclipse.cdt.managedbuilder.core/src/org/eclipse/cdt/managedbuilder/language/settings/providers/GCCBuiltinSpecsDetector.java b/build/org.eclipse.cdt.managedbuilder.core/src/org/eclipse/cdt/managedbuilder/language/settings/providers/GCCBuiltinSpecsDetector.java
index 83074435ea8..163e22a088d 100644
--- a/build/org.eclipse.cdt.managedbuilder.core/src/org/eclipse/cdt/managedbuilder/language/settings/providers/GCCBuiltinSpecsDetector.java
+++ b/build/org.eclipse.cdt.managedbuilder.core/src/org/eclipse/cdt/managedbuilder/language/settings/providers/GCCBuiltinSpecsDetector.java
@@ -80,6 +80,11 @@ public class GCCBuiltinSpecsDetector extends ToolchainBuiltinSpecsDetector imple
 			return makeList(line);
 		}
 
+		// never process #undef, see Bug 517908
+		if (line.startsWith("#undef")) {
+			return null;
+		}
+		
 		// contribution of includes
 		if (line.equals("#include \"...\" search starts here:")) {
 			state = State.EXPECTING_LOCAL_INCLUDE;