how to specify arbitrary macro variable for config.h

[John Lumby]

This is a beginner question.    I am modifying a package and would like to 
introduce a new configure option to set a value which had been previously 
hardcoded.

So  -  for example  -     I want to provide an optional configure option

    --foo-barian-characteristic=VALUE

and place a default definition in config.h.in

#undef FOO_BARIAN_CHARACTERISTIC

and then write something in configure.ac which would respond to the user 
specifying the option by replacing #undef by

#define FOO_BARIAN_CHARACTERISTIC=VALUE

in config.h

How can I do this?     I can see how by using AC_ARG_ENABLE and 
AC_DEFINE_UNQUOTED, but that would require that my option appear to be an 
--enable-xxx kind -  the user would have to specify
   configure ... --enable-foo-barian-characteristic=VALUE
which is slightly misleading since this is a setting for a default feature.

John Lumby
                                          
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