Override directive usage

[Steve Deiters]

Using make 3.81, any subsequent regular variable assignments to a
variable previously assigned using an override directive are ignored.
For example, in the following:
 
---     1969-12-31 18:00:00.000000000 -0600
+++ Makefile    2009-08-20 16:55:07.000000000 -0500
@@ -0,0 +1,6 @@
+FOO:=foo
+override FOO+=bar
+FOO+=baz
+
+all:
+       @echo $(FOO)

The output is "foo bar" instead of the expected "foo bar baz".  Is this
the expected behavior?  I would expect all regular assignments would be
valid unless overridden by values on the command line.