The newest version of pam_mount, 1.31, apparently depends on the newest version of libhx, to whit version 3.0.1. This is something of a problem, as the newest version of libhx in the main Portage tree is 2.7. I'm not sure how the new version of pam_mount got into the tree without its dependency following, but whoever migrated it from their overlay should probably also move over the new libhx.
Never mind, apparently the new version of pam_mount went into the tree a bit before the new version of libhx, and I happened to have synced in the intervening time.