crypto: allow use of nettle/gcrypt to be selected explicitly

Currently the choice of whether to use nettle or gcrypt is
made based on what gnutls is linked to. There are times
when it is desirable to be able to force build against a
specific library. For example, if testing changes to QEMU's
crypto code all 3 possible backends need to be checked
regardless of what the local gnutls uses.

It is also desirable to be able to enable nettle/gcrypt
for cipher/hash algorithms, without enabling gnutls
for TLS support.

This gives two new configure flags, which allow the
following possibilities

Automatically determine nettle vs gcrypt from what
gnutls links to (recommended to minimize number of
crypto libraries linked to)

 ./configure

Automatically determine nettle vs gcrypt based on
which is installed

 ./configure --disable-gnutls

Force use of nettle

 ./configure --enable-nettle

Force use of gcrypt

 ./configure --enable-gcrypt

Force use of built-in AES & crippled-DES

 ./configure --disable-nettle --disable-gcrypt

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
diff --git a/crypto/cipher.c b/crypto/cipher.c
index 024a00c..c8bd180 100644
--- a/crypto/cipher.c
+++ b/crypto/cipher.c
@@ -47,7 +47,7 @@
     return true;
 }
 
-#if defined(CONFIG_GNUTLS_GCRYPT) || defined(CONFIG_GNUTLS_NETTLE)
+#if defined(CONFIG_GCRYPT) || defined(CONFIG_NETTLE)
 static uint8_t *
 qcrypto_cipher_munge_des_rfb_key(const uint8_t *key,
                                  size_t nkey)
@@ -63,11 +63,11 @@
     }
     return ret;
 }
-#endif /* CONFIG_GNUTLS_GCRYPT || CONFIG_GNUTLS_NETTLE */
+#endif /* CONFIG_GCRYPT || CONFIG_NETTLE */
 
-#ifdef CONFIG_GNUTLS_GCRYPT
+#ifdef CONFIG_GCRYPT
 #include "crypto/cipher-gcrypt.c"
-#elif defined CONFIG_GNUTLS_NETTLE
+#elif defined CONFIG_NETTLE
 #include "crypto/cipher-nettle.c"
 #else
 #include "crypto/cipher-builtin.c"