configure --with-guile-for-build

[Kevin Ryde]

Below would be my suggestion to just have GUILE_FOR_BUILD for
specifying the build system guile.

In particular note that the gnu standards advise against using --with
to specify filenames, it's meant to specify an external package the
build should utilize.

Hopefully few people will need to override the default
GUILE_FOR_BUILD, and those smart enough to know what to do should be
happy with an AC_ARG_VAR variable.  Some advice could be put in the
manual if there might be any remaining doubts.

        * configure.in (--with-guile-for-build): Remove this option, it's not
        normal style for --with.
        (GUILE_FOR_BUILD): Use AC_ARG_VAR.

configure.in.guile-for-build.diff
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