patch 8.1.0420: generating vim.lib when using ActivePerl 5.20.3 or later

Problem:    Generating vim.lib when using ActivePerl 5.20.3 or later.
Solution:   Redefine XS_EXTERNAL(). (Ken Takata, closes #3462)

src/if_perl.xs

@@ -88,10 +88,8 @@
 # endif
 #endif
 
-/* Perl compatibility stuff. This should ensure compatibility with older
- * versions of Perl.
- */
-
+// Perl compatibility stuff. This should ensure compatibility with older
+// versions of Perl.
 #ifndef PERL_VERSION
 # include <patchlevel.h>
 # define PERL_REVISION   5
@@ -99,6 +97,14 @@
 # define PERL_SUBVERSION SUBVERSION
 #endif
 
+
+// Work around for ActivePerl 5.20.3+: Avoid generating (g)vim.lib.
+#if defined(ACTIVEPERL_VERSION) && (ACTIVEPERL_VERSION >= 2003) \
+	&& defined(WIN32) && defined(USE_DYNAMIC_LOADING)
+# undef XS_EXTERNAL
+# define XS_EXTERNAL(name) XSPROTO(name)
+#endif
+
 /*
  * Quoting Jan Dubois of Active State:
  *    ActivePerl build 822 still identifies itself as 5.8.8 but already

src/version.c

@@ -794,6 +794,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    420,
 /**/
     419,
 /**/