f0e5c859d36afe5 broke the build because it caused the PerlIO_funcs
declarations to be const in perlio.h and EXTPERLIO in perliol.h
and on VMS, EXTPERLIO was EXTCONST which is globalref. The compiler
considers globalref and const to be incompatible.
As a workaround, make EXTPERLIO "extern const" on VMS only. The
whole custom global data model on VMS probably needs a rethink,
but this gets the build working again.
/* Data exports - EXTCONST rather than extern is needed for Cygwin */
#undef EXTPERLIO
#ifdef PERLIO_FUNCS_CONST
-#define EXTPERLIO EXTCONST
+#ifdef __VMS
+# define EXTPERLIO extern const
+#else
+# define EXTPERLIO EXTCONST
+#endif
#else
#define EXTPERLIO EXT
#endif