XS::APItest: remove unused var

(see Message-ID: <20131121221646.GB21945@fysh.org>)
fix 'ignoring return value' compiler warnings

Various system functions like write() are marked with the
__warn_unused_result__ attribute, which causes an 'ignoring return value'
warning to be emitted, even if the function call result is cast to (void).

The generic solution seems to be

    int rc = write(...);
    PERL_UNUSED_VAR(rc);
fix Gconvert 'ignoring return value' warnings

On some systems, Gconvert() is #deffed to gcvt(), and on linux,
that function has a mandatory return value, so you get lots of
'ignoring return value' warnings. So define a V_Gconvert()
macro that does Gconvert() in a void context. Ideally this macro
would be part of the original definition of Gconvert() in config.sh,
but since Gconvert() is only used in sv.c, it was easier to
to just define V_Gconvert() locally there.