mpy-cross/Makefile: Guard "override undefine" by test for make feature.

make v3.81 doesn't have "undefine" so we can't use it with that version.

mpy-cross/Makefile

@@ -1,6 +1,7 @@
 # The following is a temporary hack to forefully undefine vars that might have
 # be defined by a calling Makefile (from recursive make).
 # TODO: Find a better way to be able to call this Makefile recursively.
+ifneq ($(findstring undefine,$(.FEATURES)),)
 override undefine COPT
 override undefine CFLAGS_EXTRA
 override undefine LDFLAGS_EXTRA
@@ -8,6 +9,7 @@ override undefine FROZEN_DIR
 override undefine FROZEN_MPY_DIR
 override undefine BUILD
 override undefine PROG
+endif
 
 include ../py/mkenv.mk