rebuild -- on or off by default

[Benjamin Kosnik]

> Rather than making a libstdc++ builddir and just building the library,
> they'll have to make a GCC builddir and regenerate large chunks of
> the compiler itself.  The "If you deleted your GCCobjdir, or never had
> one" fakeout described in gccrebuild.html won't work.  Calling the EGCS
> configure and then running "make all" (rather than "make bootstrap") works,
> but you get a lot of extra work in there if all you wanted was libstdc++
> and a rebuilt libgcc...

what about "make clean; make libgcc.a"

?

Although it does end up building tons of stuff. Oh well. I don't really 
see any other way around this.

-benjamin