Initialize GnuTLS before calling gnutls_rnd

Initialize GnuTLS before calling gnutls_rnd

* src/gnutls.c (emacs_gnutls_global_init): Now extern.
Don’t set gnutls_global_initialized if gnutls_global_init fails.
* src/sysdep.c: Include "gnutls.h", and <gnutls/crypto.h>
if 2.12 or later, which has gnutls_rnd.
(emacs_gnutls_global_init, gnutls_rnd): New fallback
placeholder macros if before 2.12.
(init_random): Initialize gnutls globals before trying to
use gnutls_rnd.