Fixes ArcticaProject/nx-libs#365

These two commits:

   commit 5c44169caed811e59a65ba346de1cadb46d266ec
   Author: Adam Jackson <ajax@redhat.com>
   Date:   Thu Mar 2 17:20:30 2017 -0500

       os: Squash missing declaration warning for timingsafe_memcmp

       timingsafe_memcmp.c:21:1: warning: no previous prototype for ‘timingsafe_memcmp’ [-Wmissing-prototypes]
        timingsafe_memcmp(const void *b1, const void *b2, size_t len)
Signed-off-by: Adam Jackson <ajax@redhat.com>

   commit d7ac755f0b618eb1259d93c8a16ec6e39a18627c
   Author: Matthieu Herrb <matthieu@herrb.eu>
   Date:   Tue Feb 28 19:18:25 2017 +0100

       Use timingsafe_memcmp() to compare MIT-MAGIC-COOKIES CVE-2017-2624

       Provide the function definition for systems that don't have it.
Signed-off-by: Matthieu Herrb <matthieu@herrb.eu>