Add --with-crypto=gnutls

Add --with-crypto=gnutls

Introduce support for GnuTLS for computing cryptograpic
hashes, similar to the OpenSSL backend.  A reason to do
this is some distributors want to avoid GPLv3, and GPG
pulls that in.

A possible extension of using GnuTLS would be replacing the GPG signing
with `PKCS#7` signatures and `X.509` keys.

We also support `--with-crypto=openssl`, which has the same effect
as `--with-openssl`, and continues to be supported.

Changes by Colin Walters <walters@verbum.org>:

 - Drop libgcrypt option for now
 - Unify buildsystem on --with-crypto

Link: https://mail.gnome.org/archives/ostree-list/2017-June/msg00002.html
Signed-off-by: Jussi Laako <jussi.laako@linux.intel.com>
Closes: #1189
Approved by: cgwalters