PATCH: Backport installed-compiler support to 3.4 branch

[Benjamin Kosnik]

> V3 people: it's my intention to backport a minimal set of
> patches necessary for testing an install libstdc++.  That will be a
> subset of all the work done on the mainline; principally the DejaGNU
> bits, but not all of the Makefile bits, as I don't want to break any
> existing testing procedures that people are using for the branches.
> Before I check anything in, I'll post and solicit comments for 24
> hours, but if you have objections to this plan in principle please let
> me know.

Sounds great.

Please, as part of this, document how install testing is supposed to work in test.html.

best,
benjamin