overriding variables?

[Alex Hornby]

In the CVS head I'm getting automake warnings if I override the value
of a variable. 

I have a common.am which is included at the top of all my Makefile.am
files which sets the value for "PERLINST" to a sensible default, but
occasionally one of the directories needs a different value, so I
override it.

e.g.
## common.am
PERLINST = perl/foo
...

## Makefile.am
include $(top_srcdir)/config/common.am
PERLINST = perl/var
...

This results in messages like:

Makefile.am:19: PERLINST multiply defined in condition TRUE

  PERLINST (User, where = 19) =
  {
    TRUE => perlidl/$(PACKAGE)/$(subdir)
  }

This used to work in 1.4. Assuming this is not a bug in CVS automake,
whats the official way to override common values nowadays?

Thanks in advance,
Alex.