* guix/utils.scm (sha256): Remove Coreutils- and libchop-based implementations. * README: Update accordingly. * m4/guix.m4: New file. * configure.ac: Use `GUIX_ASSERT_LIBGCRYPT_USABLE'. Set and substitute `LIBGCRYPT_PREFIX'. * Makefile.am (AM_DISTCHECK_CONFIGURE_FLAGS): Pass `--with-libgcrypt-prefix=$(LIBGCRYPT_PREFIX)'.