libstdc++-v3 and pthread

[Joe Buck]

> I've noticed that the latest libstdc++-v3 requires linking with pthread (at
> least on Linux). However, the g++ driver doesn't link to pthread by default.
> 
> What is the proper way ?  Fix the g++ driver to add -lpthread ?  Or require 
> the users to add -lpthread ?

The g++ driver should add all required libraries.  It won't do to ask the
user to add -lpthread as in

g++ o_file_1.o o_file_2.o -lpthread

because the searching order will be wrong; the user would have to write

g++ o_file_1.o o_file_2.o -lstdc++ -lpthread

at least, meaning that -lstdc++ is scanned twice.