crypto: fix build with gcrypt enabled

crypto: fix build with gcrypt enabled

If nettle is disabled and gcrypt enabled, the compiler and linker flags
needed for gcrypt are not passed.

Gnutls was also not added as a dependancy when gcrypt is enabled.

Attempting to add the library dependencies at the same time as the
source dependencies is error prone, as there are alot of different
rules for picking which sources to use, and some of the source files
use code level conditionals intead. It is thus clearer to add the
library dependencies separately.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20200901133050.381844-2-berrange@redhat.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Tested-by: Alex Bennée <alex.bennee@linaro.org>
Tested-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>