DEF-CALL-OUT now accepts the :GUARD argument

[<Joe...@t-...>]

Hi,

>FFI:def-call-out
>(:guard "defined(__USE_GNU)")

<Bikeshedding on>

IMHO we should have used e.g. c-guard for this and kept :guard reserved for a concept IIRC
sketched years ago: a kind dependable type, useful for variable sized arrays -- something
more on the Lisp level.
Here :guard clearly introduces C level preprocessor #ifdef, so why not use :c-* in the name,
like e.g. c-array-ptr etc.?

Regards,
	Jörg