On darwin8, no need to use flat namespace for libstdc++

[Gabriel Dos Reis]

gkeating@apple.com (Geoffrey Keating) writes:

| As of Tiger8A347, the dynamic linker is finally fixed so that new and
| delete can be overridden without needing to build both libstdc++ and
| your executable with -flat_namespace.  Yay!  We can now remove that
| workaround.  I've assumed that plain "darwin" means "not sure, please
| try to make it work in all circumstances".

Awesome!

-- Gaby