Require Libgcrypt 1.11.0

* configure.ac (NEED_LIBGCRYPT_VERSION): Set to 1.11.0
* agent/pkdecrypt.c (struct ecc_params): Move constants to the top.

It does not make anymore sense to allow building with older Libgcrypt
versions. After all PQ key support is a major feature and for this we
need Libgcrypt.