[PATCH] configure.ac: option for user-defined-trusted-dirs

[Florian Weimer]

* justinpopo6:

> From: Justin Chen <justinpopo6@gmail.com>
>
> User-defined-trusted-dirs is a variable used for adding additional
> default library search dirs for the dynamic loader. Add the ability
> to allow users to set it from the configure script.

Seems reasonable, but we should document the new option in
manual/install.texi, and regenerate the configure and INSTALL files.