Support for guile-config 0.5

[Taiju HIGASHI]

Hi,

I am currently working on the following issue.
https://issues.guix.gnu.org/55612

I have understood that I need to modify gwl to get guile-config 0.5.0
into Guix. However, I know almost nothing about gwl. Is it possible to
get some advice?

It seems that the following commits have incompatible specification
changes.
https://gitlab.com/a-sassmannshausen/guile-config/-/commit/8a107639e8b0f1037518a36fab56dde051b028d8
https://gitlab.com/a-sassmannshausen/guile-config/-/commit/3bf3adde491df551a357a3ac05d137652e825fbe

The `optional` in the configuration specification has been removed and
`argument` and `switch` are now interpreted as mandatory when `default`
is `empty`.

I have created the attached patch based on this fact. The gwl build now
passes.  Is there any other work that needs to be done? I am unsure as I
do not know much about gwl.

Thanks,

0001-config-Support-for-new-optional-mandatory-semantics.patch
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