8170784: JDK-8038957 broke cross compilation

Reviewed-by: dholmes, ihse

common/autoconf/buildjdk-spec.gmk.in

@@ -89,4 +89,10 @@ ENABLE_DEBUG_SYMBOLS := false
 # Control wether Hotspot builds gtest tests
 BUILD_GTEST := false
 
-JVM_VARIANTS := server
+JVM_VARIANTS := server
+
+# Some users still set EXTRA_*FLAGS on the make command line. Must
+# make sure to override that when building buildjdk.
+override EXTRA_CFLAGS :=
+override EXTRA_CXXFLAGS :=
+override EXTRA_LDFLAGS :=