Have the directory reader use the Unix API on VMS

opendir(), readdir() and closedir() have been available on VMS since
version 7.0.
Reviewed-by: NRich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2707)

crypto/o_dir.c

@@ -23,7 +23,8 @@
 #include "internal/o_dir.h"
 
 #define LPDIR_H
-#if defined OPENSSL_SYS_UNIX || defined DJGPP
+#if defined OPENSSL_SYS_UNIX || defined DJGPP \
+    (defined __VMS_VER && __VMS_VER >= 70000000)
 # include "LPdir_unix.c"
 #elif defined OPENSSL_SYS_VMS
 # include "LPdir_vms.c"