fix Gconvert 'ignoring return value' warnings

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.