Document building GMP/MPFR in local tree?

[Matt Fago]

Per http://gcc.gnu.org/ml/gcc/2006-10/msg00141.html it is possible to  
build GMP/MPFR in the local tree with the current Trunk. This build  
method may ease issues with building gcc. Would it be possible to  
document this for 4.3?

Thanks,
Matt