excellent description of thread model

[Benjamin Kosnik]

Discovered this buried on the gcc list yesterday, might want to think
about adding some kind of thread documentation. Right now we punt to SGI
docs for issues with actually using threads, but they are not really
applicable to understanding the underlying implementation. Perhaps I'm
missing existing documentation.

Anyway: great job with this Loren.

http://gcc.gnu.org/ml/gcc/2003-02/msg01185.html

When a port supports weak symbols and POSIX threads, libstdc++-v3 and
objc support building themselves in a mode where overhead for mutex
regions is dynamically inserted when the final link reveals that the
primary system thread library is provided non-weak.  It took multiple
years to tweak various ports to get this right.  libstdc++-v3 and objc
use the same solution; libjava uses its own threading abstraction layer
which doesn't provide this feature.  Find out why the auto-detection of
these features isn't working for OpenBSD.

-benjamin