weak for Darwin

[Aaron W. LaFramboise]

Benjamin Kosnik wrote:

> This is the right idea, however the default should be to do nothing, and
> config/os/darwin/os_defines.h should define _GLIBCXX_WEAK_DEFINITION to
> do the special stuff.

OK, but there is no config/os/darwin directory, and I have no idea what
the proper procedures are for creating one.  Anyone want to help me out? :-)

Aaron W. LaFramboise