Support for the PBKDF functions in nettle was not introduced
until version 2.6. Some distros QEMU targets have older
versions and thus lack PBKDF support. Address this by doing
a check in configure for the desired function and then skipping
compilation of the nettle-pbkdf.o module

Reported-by: Wen Congyang <wency@cn.fujitsu.com>
Tested-by: Wen Congyang <wency@cn.fujitsu.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>