Complete list of configure time options for GCC

[Joe Buck]

Ben Kosnik writes:
> Mark, there is a complete and up-to-date list of the c++ library
> configure opts here:
> 
> http://gcc.gnu.org/onlinedocs/libstdc++/configopts.html

Great!  However, since people normally build the compiler and the
library with a single "make bootstrap" command, the main installation
instructions should point to this list as well.