disable ldconfig during install

Do not bother running ldconfig on DESTDIR.  It's a waste of time as we
won't use the result (portage will rebuild the cache after install).
Also, the Gentoo sandbox does not currently catch chroot() behavior so
we end up (incorrectly) flagging it as a violation as a write to /etc.

http://sourceware.org/ml/libc-alpha/2012-08/msg00118.html
https://bugs.gentoo.org/431038