http://lists.gnupg.org/pipermail/gcrypt-devel/2012-September/001979.html
https://bugs.gentoo.org/427726

--- mpi/amd64/mpi-asm-defs.h
+++ mpi/amd64/mpi-asm-defs.h
@@ -0,0 +1,4 @@
+/* This file defines some basic constants for the MPI machinery.  We
+ * need to define the types on a per-CPU basis, so it is done with
+ * this file here.  */
+#define BYTES_PER_MPI_LIMB 8