RT3984: Fix clang compiler warning on Mac OS X where %ld is used for uint64_t.

clang suggests %llu instead, but it isn't clear that is portable on
all platforms.

C99 and above define a handy macro for us, so we try to use that
definition and fall back to current definition if needed (though we
switch to 'u' for unsigned).

Reviewed-by: Matt Caswell <matt@openssl.org>
diff --git a/apps/s_client.c b/apps/s_client.c
index 2b69355..819cff3 100644
--- a/apps/s_client.c
+++ b/apps/s_client.c
@@ -2092,7 +2092,7 @@
         ssl_print_tmp_key(bio, s);
 
         BIO_printf(bio,
-                   "---\nSSL handshake has read %ld bytes and written %ld bytes\n",
+                   "---\nSSL handshake has read %"PRIu64" bytes and written %"PRIu64" bytes\n",
                    BIO_number_read(SSL_get_rbio(s)),
                    BIO_number_written(SSL_get_wbio(s)));
     }