ssmtp-2.61-r2 (and 31) does not respect the user's (portage's) LDFLAGS. I created a patch, which you can find here: http://overlays.gentoo.org/proj/alt/browser/trunk/prefix-overlay/mail-mta/ssmtp/files/ssmtp-2.61-respect-LDFLAGS.patch
Fixed in all 2.61 ebuilds. Thanks!