Configure serialization & company

[Nathanael Nerode]

All right.

How about a note about parallel builds in the installation manual.

"If you intend to do a parallel build and you are concerned about some 
(very unlikely) race conditions in 'configure', we suggest that you do

configure
make -j1 configure-build
make -j6000 all-build
make -j1 configure-host
make -j6000 all-host
make -j1 configure-target
make -j6000 all-target"

Or something like that.  And fix up the Makefile so that one can 
actually do that (which is not much work at all).

Personally, I think that should suffice. :-/

Other people agree?

--Nathanael