cryptsetup: remove hard dependency on libgcrypt

- Add support for the alternative OpenSSL and Linux kernel crypto backends.
The latter does require that additional kernel modules (like algif_hash)
are present and loaded before invoking cryptsetup, so is used as last choice.

- Removes CRYPTSETUP_AUTORECONF as it is redundant, and reconf does
create a dependency on libgcrypt for the m4 macros used.

[Thomas: add an explicit --with-crypto_backend=gcrypt with the
libgcrypt backend is requested.]

Signed-off-by: Floris Bos <bos@je-eigen-domein.nl>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>