crypto: flip priority of backends to prefer gcrypt

Originally we preferred to use nettle over gcrypt because
gnutls already links to nettle and thus it minimizes the
dependencies. In retrospect this was the wrong criteria to
optimize for.

Currently shipping versions of gcrypt have cipher impls that
are massively faster than those in nettle and this is way
more important.  The nettle library is also not capable of
enforcing FIPS compliance, since it considers that out of
scope. It merely aims to provide general purpose impls of
algorithms, and usage policy is left upto the layer above,
such as GNUTLS.

Reviewed-by: Eric Blake <[email protected]>
Signed-off-by: Daniel P. Berrangé <[email protected]>

meson.build

@@ -823,22 +823,13 @@ if not get_option('gnutls').auto() or have_system
                       kwargs: static_kwargs)
 endif
 
-# Nettle has priority over gcrypt
+# Gcrypt has priority over nettle
 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('nettle').auto() or have_system) and not get_option('gcrypt').enabled()
-  nettle = dependency('nettle', version: '>=3.4',
-                      method: 'pkg-config',
-                      required: get_option('nettle'),
-                      kwargs: static_kwargs)
-  if nettle.found() and not cc.has_header('nettle/xts.h', dependencies: nettle)
-    xts = 'private'
-  endif
-endif
-if (not get_option('gcrypt').auto() or have_system) and not nettle.found()
+elif (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'),
@@ -852,6 +843,15 @@ if (not get_option('gcrypt').auto() or have_system) and not nettle.found()
       cc.find_library('gpg-error', required: true, kwargs: static_kwargs)])
   endif
 endif
+if (not get_option('nettle').auto() or have_system) and not gcrypt.found()
+  nettle = dependency('nettle', version: '>=3.4',
+                      method: 'pkg-config',
+                      required: get_option('nettle'),
+                      kwargs: static_kwargs)
+  if nettle.found() and not cc.has_header('nettle/xts.h', dependencies: nettle)
+    xts = 'private'
+  endif
+endif
 
 gtk = not_found
 gtkx11 = not_found