pth: fix it for (at least) gentoo multilib system.

pth: fix it for (at least) gentoo multilib system.
actually, it's not a fix, but rather an attemt to mimick its behaviour on other
systems, which is wrong, anyway, since it attempts checks against build system.
But since it works, perhaps it should do for now. On the other hand, maybe opkg
crashes somewhere around here, as gpgme depends on pth.
BTW, long time no commits ;-)