crypto: prefer gnutls as the crypto backend if new enough

If we have gnutls >= 3.6.13, then it has enough functionality
and performance that we can use it as the preferred crypto
backend.

Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>

meson.build

@@ -846,13 +846,26 @@ if not get_option('gnutls').auto() or have_system
   endif
 endif
 
-# Gcrypt has priority over nettle
+# We prefer use of gnutls for crypto, unless the options
+# explicitly asked for nettle or gcrypt.
+#
+# If gnutls isn't available for crypto, then we'll prefer
+# gcrypt over nettle for performance reasons.
 gcrypt = not_found
 nettle = not_found
 xts = 'none'
+
 if get_option('nettle').enabled() and get_option('gcrypt').enabled()
   error('Only one of gcrypt & nettle can be enabled')
-elif (not get_option('gcrypt').auto() or have_system) and not get_option('nettle').enabled()
+endif
+
+# Explicit nettle/gcrypt request, so ignore gnutls for crypto
+if get_option('nettle').enabled() or get_option('gcrypt').enabled()
+  gnutls_crypto = not_found
+endif
+
+if not gnutls_crypto.found()
+  if (not get_option('gcrypt').auto() or have_system) and not get_option('nettle').enabled()
     gcrypt = dependency('libgcrypt', version: '>=1.8',
                         method: 'config-tool',
                         required: get_option('gcrypt'),
@@ -875,8 +888,6 @@ if (not get_option('nettle').auto() or have_system) and not gcrypt.found()
       xts = 'private'
     endif
   endif
-if gcrypt.found() or nettle.found()
-  gnutls_crypto = not_found
 endif
 
 gtk = not_found