Optionally use libnettle instead of OpenSSL for crypto

OpenLDAP can be configured to be either built with OpenSSL or
GnuTLS. This commit adds support for building pw-pbkbdf2 module
without OpenSSL, by using PBKDF2 crypto primitives provided by
libnettle.
Closes hamano/openldap-pbkdf2#2

Signed-off-by: Luca Bruno <luca.bruno@rocket-internet.de>