documentation and new --enable flags

[Phil Edwards]

> > I'd like a way to feed other -f[unky] options in as well.  Right now the
> > only way is to redefine CXXFLAGS, but then we lose when rebuilding libgcc.a.
> > I suggest
>
> can the rebuild of libgcc.a take into account CXXFLAGS?

I tried this approach, and the only method I could think of that would
be certain to get the same CXXFLAGS is to (literally) grep them out of
src/Makefile*, since they aren't ever accumulated anywhere else.  This is
possible, of course, but seems kind of a hack.

However it's done, we might also want to think about saving the set of -f
options that were used to build the library.  Maybe writing them out in
a comment in bits/c++config.h -- something that would be saved with the
installed files, so that investigating a bug is easier for all concerned?


> If you can, please check in your work for the rebuild before starting on
> this. . . I'm eager to see how it works.

Done, as well as the docs (those can be backed out if needed).

Phil