Check for the presence of _WIN32 rather than its value.

Reviewed-by: NAndy Polyakov <appro@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2727)

apps/rehash.c

@@ -287,10 +287,10 @@ static int ends_with_dirsep(const char *path)
 {
     if (*path != '\0')
         path += strlen(path) - 1;
-# ifdef __VMS
+# if defined __VMS
     if (*path == ']' || *path == '>' || *path == ':')
         return 1;
-# elif _WIN32
+# elif defined _WIN32
     if (*path == '\\')
         return 1;
 # endif