Echoing --prefix choice during configure

[Brad Bell]

I would like to be able to echo the users choice for
   --prefix
while the configure script executes.

For example, the configure script documented in
   http://www.coin-or.org/CppAD/Doc/installunix.htm
currently starts with the following output
   configure: ******** Begin Checking Command Line arguments 
***************
   checking --prefix ... your choice can't be reported here (autoconf 
limitation)
   checking --with-GetStarted ... no
   checking --with-Example ... no

I notice that the discussion
  http://lists.gnu.org/archive/html/autoconf/2005-02/msg00046.html
mentions that, if the user does not specify a prefix,
it has the value NONE in the context of the configure.ac script.

I could map NONE to the default installation directory; i.e.
the directory specified by the
   AC_PREFIX_DEFAULT
before reporting it during configure  command in configure.ac.
In fact, I think that prefix (in the context of configure.ac)
should be equal to this instead of NONE.

I have not been able to find a discussion of NONE in the autoconf 
manual; i.e.,
this feature does not seem to be part of the autoconf specifications.

I do not like to use features that are not in the specifications because:
1) I do not expect such features to be portable to other systems.
2) I do not expect new versions of a program to meet such features.

I could use another syntax for prefix instead of --prefix; for example.,
  PREFIX=PrefixDir
On the other hand, this would require me to generate an error message if 
the user used
--prefix (which most users are used to using).

For now (as in the above output) I have just echoed
   checking --prefix ... your choice can't be reported here (autoconf 
limitation)