install docs updated (-enable-libstdcxx-v3 + bits)

[Benjamin Kosnik]

Hopefully, this version of the install docs is a bit easier to
understand. Less is being explained, as rebuilding libstdc++-v3
without compiler sources has now officially been deprecated. (Before,
it was just hard to do correctly. Now, it's not attempted.)

In addition, a namespace-enabled build is assumed.

I'm interested in feedback. It would be great if building v-3 became
less of a black science and more of a 'ho hum' kind of activity. 

Now, onwards.

I'd like to go ahead and assume from this point going forwards that
the --enable-libstdcxx-v3 method of configuring is standard. Is this
indeed the case, or not? If so, I'd like to re-cast two particular configure
options in a more generic way, so that people who want to use
-fsquangle, etc can still play around with user-enabled flags in some
kind of automated way. Sound good?

These are the flags in question:
--enable-namespaces [default] 
--enable-libgcc-rebuild=DIR / --disable-libgcc-rebuild 

Maybe moved to
--enable-cxx-flags=FLAGS
(which then rebuilds libgcc with FLAGS)



-Benjamin