Disable AES-NI support in Libgcrypt.
There is a problem wit VirtualBox when using AES-NI. Disabling
AES-NI for gpg-agent seems to be sufficient but we better play safe
and disable it completely.
Note that there is no problem with the Libgcrypt tests programs; even
the backported alignment test for 1.6 works fine. Needs to be
investigated further.