Pacify gcc -Wunused-macros in older GnuTLS

Pacify gcc -Wunused-macros in older GnuTLS

I ran into this problem on Ubuntu 18.04.1 LTS.
* src/gnutls.c (HAVE_GNUTLS_CIPHER_GET_IV_SIZE)
(HAVE_GNUTLS_CIPHER_GET_TAG_SIZE, HAVE_GNUTLS_DIGEST_LIST):
New macros.  This uses the same style as the other
macros that depend on GnuTLS version, as opposed to trying
to do things a bit more cleverly.
(gnutls_cipher_get_iv_size, gnutls_cipher_get_tag_size)
(gnutls_digest_list, gnutls_digest_get_name):
Define these macros only if they will be used.