Must choose one of nptl, linuxthreads

[Roland McGrath]

> How about making it possible to explicitly disable an addon?  Like
> `--enable-add-ons=no-nptl'.

I think if people really need flexibility, they can just script the ways
they form the argument for --enable-add-ons.

I checked in a different change to handle the common practical issue.  Now
add-on configure fragments can nix themselves from the add-on list.  The
linuxthreads configure fragment checks if nptl is in the list, and if so it
barfs if the list was explicit from the user and if it was the automatic
defaulting then linuxthreads just goes away so nptl can run instead.  It
would be possible to do something fancier so that e.g. --enable-kernel of <
2.6 would get you linuxthreads by default.  But we don't really need to
make it any easier to build linuxthreads.