--- old/make/hotspot/lib/JvmFlags.gmk	2019-02-05 15:45:08.038734067 +0100
+++ new/make/hotspot/lib/JvmFlags.gmk	2019-02-05 15:45:07.858734066 +0100
@@ -74,7 +74,7 @@
   endif
 else ifeq ($(DEBUG_LEVEL), fastdebug)
   JVM_CFLAGS_DEBUGLEVEL := -DASSERT
-  ifeq ($(filter $(OPENJDK_TARGET_OS), windows aix), )
+  ifneq ($call isTargetOs, windows aix), true)
     # NOTE: Old build did not define CHECK_UNHANDLED_OOPS on Windows and AIX.
     JVM_CFLAGS_DEBUGLEVEL += -DCHECK_UNHANDLED_OOPS
   endif