Fix the LIBZ macro on VC config targets

If zlib-dynamic was given but not --with-zlib-lib, LIBZ was defined to
the empty string.  Instead, give it the default "ZLIB1".
Reviewed-by: NAndy Polyakov <appro@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/1772)

Configurations/10-main.conf

@@ -1277,9 +1277,11 @@ sub vms_info {
         cflags           => "-W3 -wd4090 -Gs0 -GF -Gy -nologo -DOPENSSL_SYS_WIN32 -DWIN32_LEAN_AND_MEAN -DL_ENDIAN -D_CRT_SECURE_NO_DEPRECATE",
         defines          => add(sub { my @defs = ();
                                       unless ($disabled{"zlib-dynamic"}) {
+                                          my $zlib =
+                                              $withargs{zlib_lib} // "ZLIB1";
                                           push @defs,
                                               quotify("perl",
-                                                      'LIBZ="' . $withargs{zlib_lib} . '"');
+                                                      'LIBZ="' . $zlib . '"');
                                       }
                                       return [ @defs ];
                                     }),