on win32 translate / to \ in symlink targets

Windows, or at least NTFS, doesn't appear to follow symlinks
where the target contains the POSIX directory separator "/".

To fix that translate any / to \ in symlink targets.  This may
break code that checks the symlink target macthes a value set,
but I think it's more likely to fix code that blindly uses /
than break code that looks at the symlink target they just set.

Fixes Perl#20506

t/win32/symlink.t

@@ -74,10 +74,18 @@ close $fh if $fh;
 ok(symlink($tmpfile1, $tmpfile2), "link to it");
 ok(-l $tmpfile2, "-l sees a link");
 ok(!-f _, "-f on the lstat result is false");
-ok(-f $tmpfile2, "normal -d sees it as a file");
+ok(-f $tmpfile2, "normal -f sees it as a file");
 is(readlink($tmpfile2), $tmpfile1, "readlink works");
 check_stat($tmpfile1, $tmpfile2, "check file and link stat are the same");
 ok(unlink($tmpfile2), "unlink the symlink");
+
+# make a relative link
+unlike($tmpfile1, qr([\\/]), "temp filename has no path");
+ok(symlink("./$tmpfile1", $tmpfile2), "UNIX (/) relative link to the file");
+ok(-f $tmpfile2, "we can see it through the link");
+system "dir";
+ok(unlink($tmpfile2), "unlink the symlink");
+
 ok(unlink($tmpfile1), "and the file");
 
 # test we don't treat directory junctions like symlinks

win32/win32.c

@@ -3674,6 +3674,24 @@ win32_symlink(const char *oldfile, const char *newfile)
     */
     newfile = PerlDir_mapA(newfile);
 
+    if (strchr(oldfile, '/')) {
+        /* Win32 (or perhaps NTFS) won't follow symlinks containing
+           /, so replace any with \\
+        */
+        char *temp = savepv(oldfile);
+        SAVEFREEPV(temp);
+        char *p = temp;
+        while (*p) {
+            if (*p == '/') {
+                *p = '\\';
+            }
+            ++p;
+        }
+        *p = 0;
+        oldfile = temp;
+        oldfile_len = p - temp;
+    }
+
     /* are we linking to a directory?
        CreateSymlinkA() needs to know if the target is a directory,
        If it looks like a directory name: