fixes: I now always define SvVOK(), so depend on SvVSTRING_mg instead

and implement the skip if the vstring doeesn't compile

vstrings were originally added in perl-5.8.0-82-g92f0c26562,
SvVSTRING_mg() was originally added in perl-5.8.0-8018-gb0a11fe104 so
technically in the same release.

I expect there were some uses of intermediate versions 19 years ago,
but I don't think we need to worry about it now.

(to be squashed)

Author: tonycoz

dist/Devel-PPPort/parts/inc/magic

@@ -267,7 +267,7 @@ __UNDEFINED__ SvVOK(sv) (FALSE)
 const char *
 sv_vstring_get(pTHX_ SV *sv, STRLEN *lenp)
 {
-#ifdef SvVOK
+#ifdef SvVSTRING_mg
     MAGIC *mg = SvVSTRING_mg(sv);
     if (!mg) return NULL;
 
@@ -776,6 +776,8 @@ is tied($big)->{store}, 0;
 SKIP:
 {
     my $vstr = eval "v1.23.456";
+    $vstr
+        or skip "No vstrings", 2;
     ok Devel::PPPort::SvVOK($vstr), '$vstr has SvVOK';
     is Devel::PPPort::SvVSTRING($vstr), "v1.23.456", 'SvVSTRING()';
 }